Au Schottky- and p–n junction-diode detectors formed by backside laser irradiation doping

Nuclear Inst. and Methods in Physics Research, A 985 (2021) 164683

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Comparative study of In/CdTe/Au Schottky- and p–n junction-diode detectors formed by backside laser irradiation doping Junichi Nishizawa a , Volodymyr Gnatyuk b,c ,∗, Kateryna Zelenska c , Akifumi Koike c , Toru Aoki a,c a

Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan V.E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Prospekt Nauky 41, Kyiv 03028, Ukraine c Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan b

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Keywords: CdTe crystal Schottky diode Backside laser irradiation doping p–n junction diode X/𝛾-ray detector I–V characteristics Isotope emission spectra Thermal tolerance

ABSTRACT Two types of In/CdTe/Au diode structures were fabricated using detector-grade p-CdTe single crystals: (i) by vacuum evaporation of In and Au contacts on the chemically treated (111) surfaces; (ii) by backside laser doping when the In/CdTe structures were irradiated from the crystal side with a wavelength for which the semiconductor was transparent. Thus, diodes with a Schottky contact (In/CdTe) and a p–n junction were obtained, respectively. To prove the doping of the thin region near the In/CdTe interface by In (donor) and study thermal tolerance of both types of In/CdTe/Au diode detectors, room temperature I–V characteristics and 57 Co isotope emission spectra were measured before and after annealing of the diodes at temperatures below and above the In melting point. Thermal annealing at lower temperatures led to a slight increase and decrease in reverse dark current of the unirradiated and laser-irradiated samples, respectively. Heating up to 200 ◦ C resulted in a significant increase in reverse current and complete spectra degradation in the samples fabricated without laser processing. After such annealing, the electrical characteristics of the p–n junction diodes, formed by the backside laser doping technique, became optimized and 57 Co spectra were almost unchanged. It was supposed that a Schottky barrier at the In/CdTe interface was degraded, while a p–n junction, created in a deeper region of the CdTe crystal, remained functional even after melting and solidifying the In contact.

1. Introduction Since discovering the ability of CdTe to be sensitive to high-energy ionizing radiation (1950s) [1,2], the second stage in the development and application of this semiconductor as X/𝛾-ray detectors has been due to designing them as diode structures (1960s) [1–5]. Diode-type detectors, operating in reverse mode, have shown much higher energy resolution compared with ohmic-type detectors owing to fuller collection of photogenerated charge carriers achieved by applying higher bias voltages [1–5]. Generally, two kinds of CdTe-based detector diode structures have been developed: (i) with a Schottky barrier (one contact is quasi-ohmic and second is rectifying) [1,2,5–9], (ii) with an electrical junction (in particular, a shallow abrupt built-in p–n junction) [1–4]. Intensive and successful development of CdTe-based X/𝛾-ray detectors formed as Schottky diodes began since the end of 1990s, when Acrorad Co. obtained high quality semi-insulating CdTe crystals and used a low work-function metal (indium) to create a high Schottky barrier for holes on the crystal surface that provided high energy resolution of In/CdTe/Pt diode X/𝛾-ray detectors [5]. Along with In, Al

has been widely used as a Schottky contact to Acrorad’s CdTe crystals [6–9]. Al has the higher melting temperature than In, therefore this electrode metal is more suitable for solder bumping and wire bonding procedures which are applied in pixelated detectors, in particular under assembling large sensor systems by tiling the pixel detectors, 3D chip stacking, etc. Al/CdTe/Pt detectors allow the pixelization of the anode electrode that is important for the development of electronsensing detectors [7]. Nowadays, Al/CdTe/Pt and In/CdTe/Pt Schottky diodes are the main industrial high-resolution uncooled semiconductor X/𝛾-ray detectors [9]. Another type of CdTe diode detectors is that based on semiconductor structures with an electrical junction [1–4]. M-p–n structured diodes have particular advantages over Schottky diodes such as higher reliability of electrical contacts, temperature stability and resistance to damage of the barrier structure because an electrical barrier is located inside the CdTe crystal (under the thin doped surface layer). Furthermore, in case of formation of a Schottky contact (In/CdTe or Al/CdTe interface), very high requirements are imposed on the CdTe crystal surface state, therefore, various techniques of semiconductor surface processing are

∗ Corresponding author at: V.E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Prospekt Nauky 41, Kyiv 03028, Ukraine. E-mail addresses: [email protected] (J. Nishizawa), [email protected] (V. Gnatyuk), [email protected] (K. Zelenska), [email protected] (A. Koike), [email protected] (T. Aoki).

https://doi.org/10.1016/j.nima.2020.164683 Received 12 August 2020; Received in revised form 18 September 2020; Accepted 19 September 2020 Available online 22 September 2020 0168-9002/© 2020 Elsevier B.V. All rights reserved.

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Nuclear Inst. and Methods in Physics Research, A 985 (2021) 164683

In/CdTe structures and doping of the thin CdTe region was a reason of relatively low yield of the M-p–n structured In/CdTe/Au diode detectors with acceptable parameters [16]. Therefore, in addition to frontside irradiation used before [14– 20], we have investigated the doping technique based on backside laser irradiation allowing to create extreme conditions in the confined area at the CdTe–metal (dopant) interface and thus, to introduce and electrically activate high concentration of dopant atoms in the nearinterface semiconductor layer that results in formation of a high barrier p–n junction in the surface region of the CdTe crystal [27–31]. Backside laser irradiation of semiconductors with a wavelength for which the semiconductor is transparent has been studied long time and applied for modification of their structure, electrical and optical properties [32,33], and also for solid-state doping [21–24]. Moreover, such kind of irradiation of detector-grade CdZnTe crystals, transparent for employed laser pulses (𝜆 = 1064 nm), resulted an increase in resistivity and improvement of optical properties of the semiconductor [32,33]. This was associated with redistribution of impurities atoms and intrinsic defects in the crystal bulk due to the temperature gradient field around Te inclusions, which played the role of absorption centers at laser irradiation. X/𝛾-ray detectors, based on laser-irradiated CdZnTe samples demonstrated higher efficiency and energy resolution [32]. We studied several approaches of laser-induced doping of detectorgrade CdTe semiconductor produced by Acrorad Co., Ltd. [9], where pulses of a KrF excimer (𝜆 = 248 nm, 𝜏 = 20 ns) laser [14–19], ruby (𝜆 = 694 nm, 𝜏 = 20 ns) laser [20] or YAG:Nd (𝜆 = 1064 nm or 532 nm, 𝜏 = 8 ns) laser [27–31] were employed to treat CdTe–metal (dopant film) structures using frontside [14–20,30,31] or backside [27– 31] irradiation in different environments (vacuum, ambient air, high pressure argon or water). We have continued to study the unique ability of pulsed laser processing to dope semiconductors with concentrations far higher their equilibrium solubility limit avoiding formation of impurity-defect complexes, secondary phases and self-compensation. Based on our research results and other relevant investigations referred above, we have chosen backside laser irradiation (𝜆 = 1064 nm) of In/CdTe structures in ambient air to dope the thin p-CdTe region with In (donor) atoms, form a shallow abrupt p–n junction and create diode-type X/𝛾-ray detectors. A particular emphasis has been made on the details and analysis of the doping technique based on backside laser irradiation of metal–semiconductor structures because this promising technological procedure will be developed and optimized in our future investigations.

studied to employ optimal ones: chemical etching, thermal annealing, plasma treatment, laser irradiation, ion bombardment, etc. [5–9]. For creation of a built-in electrical junction, the doping technique is of greater importance to form a thin heavily doped semiconductor region and, thus a shallow abrupt p–n junction. However, efficient doping of high-resistivity CdTe has still remained a critical challenge, limiting the performance of CdTe-based X/𝛾-ray detectors and other devices [10, 11]. We have succeeded in the development of both types of CdTebased diode detectors in particular, we have achieved high energy resolution (FWHM of the 662 keV peak in the 137 Cs spectrum at room temperature) in Schottky diodes (0.4–1.0%) [12–14] and diodes with a p–n junction (0.7–1.5%), fabricated by the laser-induced doping technique [14–16]. In the last case, high-resistivity p-like CdTe crystals, pre-coated with an In dopant film, were irradiated with nanosecond laser pulses from the front side (In film) then, the second electrode (Au) was evaporated in vacuum onto the opposite side (CdTe surface) of the formed In/CdTe structure [14–20]. The laser doping method using backside laser irradiation (when the dopant-semiconductor structure is irradiated from the semiconductor side with a wavelength for which the semiconductor is transparent) has not been sufficiently studied, although it is very promising because this technique allows to achieve extremely fast heating and melting of very thin dopant and semiconductor layers in the confined area at the interface [21–24]. This results in generation of high pressure and stresses, stimulates abnormally fast diffusion and mass transfer, high solubility and segregation, and other highly non-stationary non-equilibrium processes which occur in the confined area and open possibilities to obtain a thin heavily doped semiconductor region [21]. In the present paper, the backside laser irradiation doping technique has been studied based on the measurements of electrical and spectroscopic properties of the developed CdTe p–n junction diode detectors which are compared with the corresponding characteristics of the Schottky diodes created on identical CdTe crystals using the same surface processing and metals for electrode deposition but without laser irradiation. 2. Analysis of laser doping techniques Pulsed laser-induced doping of semiconductors makes it possible to obtain substitutional dopant concentration which far exceeds the equilibrium solubility and one more advantage of such kind of laser processing is very high electrical activation of dopant atoms [21]. The efficiency of laser processing is enhanced when irradiation is carried out in liquid [16]. The concentration profile of incorporated dopants strongly depends on duration and energy density of laser pulses as well as laser irradiation conditions. Using longer or shorter laser pulses and action from the deposited dopant film side (frontside irradiation) or semiconductor side (backside irradiation) in various environments (vacuum, high pressure gas or liquid) allows to obtain different distributions of intrinsic defects, background impurities or dopants in thicker or thinner semiconductor regions and hence, to form an electrical junction in the crystal at different depths [15–26]. Therefore, laserstimulated doping is considered as a unique tool for heavy doping of thin layers of semiconductors and formation of abrupt built-in electrical junctions. However, the employed frontside laser irradiation technique for doping of the thin semiconductor region near the CdTe–metal (dopant) interface has some features like lack of photo- and thermal (in case of a thick dopant film) effects on the interface and only the actions of laser-induced pressures and stresses are utilized [14–20,25,26]. This can be as an advantage (to avoid photo- and thermo-processes) or disadvantage for the doping procedure. In particular, the absence of a thermal effect on the CdTe–metal (dopant) interface in some cases does not allow achieving optimal conditions for laser-stimulated doping and creation of an abrupt p–n junction. Moreover, low controllability of laser action directly on the interface under frontside irradiation of

3. Experimental details on formation and measurements of CdTe diodes 3.1. Semiconductor samples During the last two decades, we have intensively and successfully employed commercial detector-grade Cl-compensated p-like CdTe semiconductor, produced by Acrorad Co., Ltd. using the Traveling Heater Method (THM) [9], to investigate electrical and photoelectric properties, develop Schottky and p–n junction diodes, study of charge carrier transport processes in the fabricated diode structures and examine them as X/𝛾-ray detectors [12–20,27–31]. THM is an efficient technique to grow uniform CdTe single crystals with decreased number of native point and extended defects, and accidental impurities [1,2,5]. Semiinsulating CdTe crystals with weak p-type conduction showed room temperature resistivity 𝜌 ≈ 5 × 109 Ω⋅cm that was close or even higher than the intrinsic value (𝜌i ≈ 4 × 109 Ω⋅cm) thus, the employed CdTe was considered as an almost intrinsic semiconductor with p-like type conductivity [13]. The (111) oriented single-crystal wafers cut from a CdTe ingot were preliminary polished by the manufacturer. Parallelepiped-like samples with a surface area of 5 × 5 mm2 and thickness of 0.75 mm 2

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were used for the experiments (formation of Schottky diodes, laserinduced doping, formation of p–n junction diodes and X/𝛾-ray detector fabrication). Before electrical contact or dopant film deposition, laser irradiation, and electrode formation, all the CdTe samples were subjected to chemical surface processing for cleaning from contaminants (washing in acetone and methanol), removing the disordered surface region including oxide films (etching in a polishing Br–methanol solution during 90 s) and final washing (thorough rinsing in a few beakers with methanol). Indium film (as a rectifying contact or doping source) was immediately deposited after the chemical treatment of the CdTe crystals to minimize surface oxidation effects.

region adjacent to the interface and activation of them as donor centers [27–31]. The formation of a thin n-type layer resulted in creation of a p–n junction in the CdTe crystal near the metal–semiconductor interface. After backside laser irradiation of the In/CdTe structures with a certain number of laser pulses, the samples were subjected to passivation in an aqueous H2 O2 solution for 40 s and then, were thoroughly rinsed in pure methanol. Hydrogen peroxide passivation was carried out to modify the surface state of the CdTe crystals before forming the second electrical contact (Au electrode) on the opposite side of In/CdTe structures, decrease the lateral surface component of dark current and stabilize the electrical parameters of the In/CdTe/Au diodes. An Au electrode was deposited on the CdTe(111)A (Cd-terminated) surface with the same area and thickness by the same deposition technique (thermally evaporation in vacuum) as in the case of the In film (electrode). It was assumed that a ‘‘quasi-ohmic’’ contact was formed at the Au/CdTe interface [15–17]. Thus In/CdTe/Au p–n junction diodes were obtained. It is known that deposition of an In electrode onto a CdTe crystal creates a Schottky barrier [1,2]. This is successfully used in the fabrication of high-energy resolution X/𝛾-ray detectors based on formation of In/CdTe Schottky diodes [5]. It is a challenge to distinguish detectors fabricated as Schottky diodes or p–n junction diodes and decide what type is optimal for X/𝛾-ray sensors. Therefore, we have also investigated In/CdTe/Au Schottky diode detectors formed in the same manner as In/CdTe/Au p–n junction diode detectors but without the procedure of laser irradiation.

3.2. Laser-induced doping and creation of a p–n junction Indium is a highly favorable n-type dopant in semi-insulating plike CdTe because of its low defect formation energy and shallow transition energy level in the semiconductor [10,11]. However, efficient doping of high-resistivity CdTe is a tricky problem, moreover the mechanism of the low dopability in CdTe is not well understood although it is attributed to rather low solubility of In in CdTe (or CdTe in In) and spontaneous formation of oppositely charged native defects or defect complexes of the dopant (e.g., DX- or AX-centers) as the Fermi energy level shifts with increasing charge carrier density. The last factor limits further change of the Fermi energy (i.e., the Fermi level is pinned) [10,11]. However, solubility of the donor dopant (In) can significantly increase and formation of acceptors complexes (e.g., VCd -InCd ) can be suppressed under extremely non-equilibrium and non-stationary conditions of pulsed laser irradiation (fast heating, melting, high pressure and stress, quick cooling, etc.) [24–31]. Thus, indium was chosen as a donor dopant for semi-insulating p-like CdTe crystals. An In dopant film was thermally evaporated in vacuum onto the CdTe(111)B crystal surface (Te-terminated) using a Mo mask of the size of 4 × 4 mm2 without heating the sample. A mask was applied to avoid getting indium on the lateral sides of the crystal. The In film thickness was estimated to be about 200 nm to provide the optimal conditions for induced doping of the semiconductor region near the In/CdTe interface under backside laser irradiation. The film was not evaporated even at multiple irradiation, so it served as both an n-type dopant source under laser action and electrode after formation of the p–n junction diodes. The formed In/CdTe structures were subjected to irradiation with nanosecond pulses of an infrared laser from the CdTe(111)A side (Cdterminated) in air at room temperature. The experimental setup of backside laser irradiation is schematically shown in Fig. 1. A YAG:Nd laser with wavelength 𝜆 = 1064 nm and pulse duration 𝜏 = 20 ns operating in a frequency mode with the repetition rate of 10 Hz was employed. In order to provide uniform irradiation of the whole area of the In/CdTe interface through the CdTe crystal, i.e. entire area of the deposited dopant (In film), a beam expander and homogenizer were introduced into the light path of the optical system which also included a special wavelength-selective mirror selector, color and neutral density optical filters to avoid illumination of the sample by the second harmonic (𝜆 = 532 nm) of a YAG:Nd laser and attenuate radiation with 𝜆 = 1064 nm, respectively (Fig. 1). The pulse energy density was varied in a wide range E = 5–100 mJ/cm2 and number of laser shots, used in the experiments, was N = 10–100. At such laser parameters, the samples had time to cool between pulses, hence the heat accumulation effect under irradiation was negligible. Backside laser irradiation of the In/CdTe structures was resulting in strong absorption of the radiation only by a thin In layer (absorption coefficient is ∼106 cm−1 ) at the In/CdTe interface because of CdTe semiconductor is almost transparent for the employed laser wavelength (absorption coefficient is ∼18 cm−1 ). It was supposed that direct laser impact on the metal–semiconductor interface (extreme fast heating and melting of an thin In layer in the confined area, generation of stresses in the crystal, quick cooling of the interface region, etc.) provided particular conditions for simulated incorporation of In atoms into the CdTe

3.3. Measurement techniques of diode detectors The developed In/CdTe/Au diode-type detectors were examined by room temperature measurements of the electrical characteristics and detection of the 57 Co isotope emission spectra. The properties of the diodes obtained by laser-induced doping with different irradiation parameters were analyzed and compared with the characteristics of the Schottky diodes created on the similar CdTe crystals using the same surface processing and metals for electrode deposition as for the p–n junction diodes. A feature of the research was monitoring of I–V characteristics and isotope spectra measured by the diode detectors after low-temperature annealing. The measurements were carried out before and after the samples were heated up to 50 ◦ C, 100 ◦ C, 150 ◦ C, and 200 ◦ C then, cooled back to room temperature. Heating of the In/CdTe/Au diode structures was carried out in a chamber under a vacuum of 3.5 × 10−4 Pa. The annealing temperature was reached in 10–20 min, the samples were kept heated for 60 min. After annealing, the samples were taken out and cooled down to room temperature naturally. The heating procedure allowed us to evaluate whether a p–n junction in the CdTe crystals was formed after laser irradiation or rectifying characteristics of the In/CdTe/Au structures were due to just a Schottky barrier at the In/CdTe interface. Moreover, thermal tolerance of the detectors with a p–n junction and Schottky contact was monitored. The I–V characteristics, measured in dark conditions, evidenced high rectification properties of the obtained In/CdTe/Au diodes. Reverse current flowed when the In electrode was biased positively with respect to the Au electrode (quasi-ohmic contact). Diode-type X/𝛾ray detectors operate in reverse bias mode and respond to incident photons [2], therefore it is important that the fabricated In/CdTe/Au diode structures with a Schottky barrier or p–n junction have low reverse dark current (leakage current). Low leakage current allowed to apply higher bias voltages to extend the depletion region up to the whole thickness of a CdTe crystal and thus achieve better or even full collection of photogenerated charge carriers and hence obtain higher energy resolution either in Schottky [12–14] or p–n junction [14–17] diode detectors formed on CdTe crystals similar to those we used in the present experiments. 3

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Fig. 1. Schematic illustration of the doping technique using backside laser irradiation.

The experimental setup of the measurements of isotope spectra is shown in Fig. 2. Both types of In/CdTe/Au diodes (fabricated without laser processing and using backside laser irradiation) were tested as X/𝛾-ray detectors by the same measurement technique. A sample was placed into a shielded box which was used to reduce electrical noises and avoid daylight illumination. The In and Au electrodes were connected with coaxial connectors to the signal line and shield, respectively. The In electrode was biased positively with respect to the Au one using a power supply source High Voltage Standard HSX-3R5 (Matsusada Precision Inc.). An In/CdTe/Au diode detector was exposed to X/𝛾-rays from the In electrode side using a 57 Co isotope as a highenergy radiation source (Fig. 2). A photogenerated signal, collected by a Preamplifier 5102, was delivered to a Pulse Shaping Amplifier 4419 HI (both were manufactured by CLEAR-PULSE Co., Ltd) through a 10 dB attenuator. The shaping time was optimally estimated to avoid ballistic deficit and limit electrical noise. The risetime (carrier drift time) for p-type semiconductors depended on the electric field strength, hole mobility and thickness of the crystal and it was calculated as ∼150 ns using charge transport properties data for semi-insulating p-CdTe:Cl grown by the THM [34]. Therefore, the constant for the pulse shaping amplifier was chosen as 2 μs that was more than 10 times longer than the pulse risetime. Shorter shaping time could lead to errors due to ballistic deficit but longer shaping time increased electrical noises. The output signal from the shaping amplifier was processed by a multichannel analyzer MCA 7700 (Seiko EG&G) (Fig. 2). A diode detector was kept biased at high voltage in the dark for 5 min before the measurements. The employed electronic devices were adequately calibrated to ensure correct channel positions in the isotope spectra taken by a detector. There are the optimal bias voltages applied for CdTe-based diodetype detectors either with a Schottky contact or p–n junction that provide better signal-to-noise ratio and, therefore, higher detectability and energy resolution [13–15]. Reverse bias voltages V = 50–200 V were applied for both types of In/CdTe/Au diode detectors to measure the emission spectra of a 57 Co isotope. The distance between the radiation source and detector was fixed during the spectral measurements which lasted 1 min.

not been sufficiently well determined. Perhaps, for each laser energy density there is the optimal number of pulses that allows achieving the lowest reverse dark current in I–V characteristics of created p–n junction In/CdTe/Au diodes. 4.1. I-V characteristics of In/CdTe/Au diodes Initially, we used the values of laser pulse energy density E close to those that led to the excellent results in the case of frontside laser irradiation [14–20]. As it turned out, in the case of backside laser irradiation, i.e. direct laser impact on the In/CdTe interface, lower values of E were sufficient to achieve optimal diode detector parameters. Fig. 3 shows forward (a) and reverse (b) branches of the room temperature I– V characteristics of the In/CdTe/Au diodes just with a Schottky contact, i.e. unirradiated samples (curves 1 and 2), and with a p–n junction, formed by backside laser irradiation with sets of 30 pulses of different energy densities E, i.e. lase-irradiated samples (curves 3–6). As seen (Fig. 3, curves 1 and 2), the In/CdTe/Au structures, fabricated without laser irradiation, demonstrate quite high rectification properties due to a high Schottky barrier at the In/CdTe interface that is general and typical for an electrical contact of In and semi-insulating CdTe and it has been successfully used for Schottky diode detector fabrication [5,9]. However, the In/CdTe/Au structures, prepared using backside laser irradiation, exhibit steeper I–V characteristics (Fig. 3, curves 3–6). Forward biased currents are generally higher (Fig. 3(a), curves 3–5) and reverse ones are lower (Fig. 3(b) , curves 3–6) in the irradiated In/CdTe/Au samples (p–n junction diodes) compared with the corresponding values of the unirradiated ones which are considered as In/CdTe/Au Schottky diodes (Fig. 3, curves 1 and 2). Energy density 𝐸 = 14 mJ/cm2 was the optimal value for backside irradiation doping with 𝑁 = 30 laser pulses to achieve the best rectification efficiency of the In/CdTe/Au diodes, i.e. obtain highest forward current (I ≈ 87 μA at 𝑉 = 30 V) (a) and lowest reverse one (I ≈ 1.4 nA at V = −200 V) (b) (Fig. 3, curves 4). However, it also depends on other irradiation parameters and conditions (duration of laser pulses, repetition rate, ambient conditions, In dopant film thickness, etc.) [27–31]. It was possible to obtain high rectification properties of the In/CdTe/Au structures using different irradiation parameter sets, for example, with smaller E and larger N and vice versa. As seen, the I–V characteristics of both types of In/CdTe/Au diodes are quite similar that can evidence almost the same heights of Schottky contact and p–n junction barriers (Fig. 3). A little difference in the values of reverse currents in these diodes can be due to the bias voltage dependence of the Schottky barrier height (it is decreasing with increasing the electrical field of reverse polarity) [35] as well as an increase in the resistivity of the bulk part of the CdTe crystal after backside laser irradiation [32,33]. A laser-induced increase in resistivity of detector-grade CdZnTe crystals was observed under irradiation with the similar nanosecond laser pulses (𝜆 = 1064 nm). Despite the feature that Cd(Zn)Te semiconductors are transparent for such radiation, selective absorption by Te inclusions in the crystal causes temperature gradient fields, stimulating compensation of Cd vacancies with Cd interstitials

4. Measurement data and analysis Since one of the key requirements for diode-type X/𝛾-ray detectors, operating under reverse bias, is low reverse dark current (leakage current), it was important to monitor I–V characteristics of the fabricated In/CdTe/Au diodes. Studies of the electric properties of the In/CdTe/Au samples, irradiated with different numbers of laser pulses N, carried out in a wide range of energy densities E, revealed the features associated with the critical dependence of I–V characteristics on both the laser radiation parameters N, E and 𝜏, and radiation dose 𝐷 = 𝑁⋅𝐸⋅𝜏. Despite the fact that a large number of CdTe crystals pre-coated with an In dopant film were subjected to backside laser irradiation with different energy densities and numbers of pulses and then were tested by I–V measurements, the optimal set of the laser irradiation parameters has 4

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Nuclear Inst. and Methods in Physics Research, A 985 (2021) 164683

Fig. 2. Schematic illustration of the experimental setup of the isotope spectrum measurements with In/CdTe/Au diode detectors.

Fig. 3. Forward (a) and reverse (b) branches of the I–V characteristics of the In/CdTe/Au diode detectors fabricated without laser irradiation (Schottky diodes) (curves 1 and 2) and with the backside laser doping technique ( p–n junction diodes) (curves 3–6) using 30 pulses with energy density E (mJ/cm2 ): 6.4 (curves 3), and 14 (curves 4), 19 (curves 5), and 65 (curves 6).

distinctive feature of the fabricated In/CdTe/Au structures. This was an important evidence of formation of different barriers at the In/CdTe interface in the case of just vacuum evaporation of an In film on the CdTe surface and when this interface was irradiated with nanosecond laser pulses through the CdTe crystal. The In/CdTe/Au Schottky diodes suffered from heating, in particular at higher temperatures, whereas the p–n junction diodes with the same electrodes showed improved electrical performance after thermal annealing, in particular reverse dark current decreased (Fig. 4).

and impurity atoms and thus, decreasing the concentration of intrinsic electrical active point defects VCd [33]. It was interesting to find out thermal tolerance of electrical properties of both types of In/CdTe/Au barrier structures (Schottky and p–n junction diodes). Figs. 4(a) and (b) show the room temperature I–V characteristics of the unirradiated sample and diode formed using backside laser irradiation, respectively, which were subjected to thermal exposure at 𝑇 = 50 ◦ C (curves 1), 𝑇 = 100 ◦ C (curves 2), 𝑇 = 150 ◦ C (curves 3), and 𝑇 = 200 ◦ C (curves 4). After annealing, the samples were cooled down to room temperature naturally and then measured. It was revealed that reverse dark currents (leakage currents) did not change much in the samples after heating them up to temperatures below the melting point of indium (𝑇In = 156.75 ◦ C). However, different trends were observed for the unirradiated and laser-irradiated In/CdTe/Au structures. In the first case, reverse current increased (in 1.2–2.1 times), whereas in the second case, it decreased (in 1.2–1.4 times) in the samples annealed at 50 ◦ C, 100 ◦ C, and 150 ◦ C (curves 2–4 in Fig. 4(a) and (b), respectively). Another situation was observed when the annealing temperature was higher than the In melting point (T > 𝑇In ), i.e. the In contacts were melted then solidified after cooling of the In/CdTe/Au diodes. In the case of unirradiated samples, heating to 𝑇 = 200 ◦ C resulted in deterioration of the I–V characteristics. The reverse dark current increased in 7–10 times and electrical noise remarkably increased, too (Fig. 4(a), curve 5). Whereas the In/CdTe/Au structures, obtained by backside laser irradiation, demonstrated a further decrease in reverse current after such annealing (𝑇 = 200 ◦ C) (Fig. 4(b), curve 5). Severe degradation of the electrical performance of the unirradiated diodes and optimization of the electrical characteristics of the laserirradiated diodes after the same thermal annealing at T > 𝑇In was the

4.2. Isotope spectra measured by In/CdTe/Au diodes Dozens of In/CdTe/Au diodes were tested as X/𝛾-ray detectors by emission spectra measurements of different isotopes. It was established that the samples, fabricated without laser irradiation, showed similar spectroscopic characteristics which depended on the regimes of chemical surface processing of CdTe crystals before In and Au electrode deposition. In the case of laser-irradiated samples, the detection efficiency (number of counts) and energy resolution (FWHM) in the isotope emission spectra were strongly dependent on the laser pulse energy density E or number of laser shots N under backside irradiation of the In/CdTe structure with fixed E. Certainly, the shape of the energy spectra depended on applied bias voltage and measurement time, however these parameter were kept the same for both types of In/CdTe/Au diodes when the spectral measurements of emission from the same isotopes were carried out. The spectroscopic characteristics of two types of In/CdTe/Au diode detectors, prepared using the same procedures and conditions of the preliminary surface treatments, passivation and contact (electrode) deposition were also similar. In this study, the spectroscopic performance of In/CdTe/Au diode detectors was monitored by measurements of the emission spectra of 5

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Fig. 4. Room temperature reverse biased I–V characteristics of the In/CdTe/Au diode detectors fabricated without laser irradiation (Schottky diodes) (a) and with the backside laser doping technique ( p–n junction diodes) (b) before (curves 1) and after annealing at temperature T (◦ C): 50 (curves 2), 100 (curves 3), 150 (curves 4), and 200 (curves 5).

a 57 Co isotope using the technique and experimental setup described above (Fig. 2) at room temperature without risetime discrimination or pulse height correction electronics. Despite the fact that, reverse dark current of the In/CdTe/Au structures, fabricated without laser irradiation and heated at temperatures lower than the melting point of the In contact, slightly increased compared with the samples which were not subjected to thermal annealing (Fig. 4(a), curves 1–4), it remained low enough and such diodes were sensitive to isotope radiation and could be used as X/𝛾-ray detectors. Both types of In/CdTe/Au diodes with relative low reverse dark currents (I < 20 nA at V = −200 V) were sensitive to X/𝛾-rays and recorded similar isotope spectra. Fig. 5 (curves 1) shows the room temperature emission spectra of a 57 Co isotope taken with the Schottky diode (a) and p–n diode (b) detectors in the same conditions at applied reverse bias voltage of 200 V. Both types of detectors clearly resolve the major 57 Co 122 keV peak, although another high-energy peak (at 136 keV) is buried in the noise. A quite symmetrical shape of the 122 keV band in the spectra indicates that the full charge collection is achieved even at not so high bias voltage (V = −200 V). A broad shoulder (low-energy tail) to the left side of the 122 keV peak is associated with many physical processes, including Compton scattering [1,2,5]. The distortion (tail structure) in the isotope spectra is attributed to background continuum and Compton scattering of 𝛾-rays in the surrounding materials, however it can also result from not high enough mobility-lifetime product of charge carriers in CdTe, especially for holes [2,5]. The steep rise in the left part of the spectra is due to electronic noise (Fig. 5 curves 1). As seen, the detection efficiencies and energy resolutions of both types of the detectors were almost the same: the height of the 122 keV photopeaks ∼25 counts and FHWM = 11.5 % in the 57 Co isotope spectra (Fig. 5 curves 1). The spectra measured after heating and annealing of the In/CdTe/Au diode detectors at temperatures of 50 ◦ C, 150 ◦ C, and 150 ◦ C did not change much. The 57 Co isotope spectra measured by both types of In/CdTe/Au diode detectors, annealed at T < 𝑇In , were a little worse or better than those shown in Fig. 5 (curves 1). Slight differences in the spectroscopic characteristics were attributed to the initial detector performance (structure inhomogeneities in different crystals, deviations in surface processing, laser-induced doping, contact formation, etc.). The most interesting and important result was obtained when the spectroscopic measurements were carried out for the In/CdTe/Au diode detectors subjected to higher temperature annealing, i.e. at T > 𝑇In . Heating and annealing of the samples, fabricated without laser processing, at 𝑇 = 200 ◦ C resulted in loss of 𝛾-ray sensitivity; as seen,

the 𝛾-peak from a 57 Co isotope was obscured by the noise (Fig. 5(a), curve 2). It was impossible to provide the spectra measurement at bias 𝑉 = −200 V, used for the sample before annealing, because of a low photogenerated signal compared to the electronic noise. Only at 𝑉 = −50 V, the very little peak (at 122 keV) was hardly distinguished in the 57 Co isotope spectrum, measured by the annealed In/CdTe/Au Schottky diode detector (Fig. 5(a), curve 2). Such annealing (T > 𝑇In ) of the In/CdTe/Au diode detectors, created by backside laser irradiation doping led to a rise of the low-energy wing of the 57 Co isotope spectrum and to a slight decrease of the 122 keV peak height (Fig. 5(b), curve 2). However, the peak was clearly distinguished with a little higher energy resolution (FWHM = 11.0 %) and the p–n junction diode detectors remained functionality. 5. Discussion of the results 5.1. Problem of distinguishing a laser-induced p–n junction or laser-modified Schottky barrier For fabrication of both types of diode detectors, an In film was deposited in the same manner onto the CdTe(111)B crystal surface, preliminary subjected to chemical etching and polishing. This is the standard procedure of formation of a Schottky contact to detectorgrade CdTe that has been widely used by researchers and manufacturers [1,2,5,9]. Then, a number of the prepared In/CdTe structures were irradiated from the crystal side with a wavelength for which the semiconductor was transparent to directly impact the In film and activate a thin In layer as a dopant source. It was assumed that such procedure introduced In atoms into the semiconductor, provided doping of a thin layer of p-like CdTe with an In impurity (donor), and thus formed an abrupt p–n junction inside the crystal near the In/CdTe interface. The second (quasi-ohmic contact) was formed using the same technique for both types of diode structures, i.e. just by vacuum deposition of an Au electrode onto the CdTe(111)A face opposite to the In-coated one. The difference in the fabrication of the In/CdTe/Au diodes was only in employing backside laser irradiation of the In/CdTe structures before the Au contact deposition to form p–n junction diodes. Unirradiated In/CdTe/Au structures were considered as Schottky diodes. One of the purposes of using the same technological procedures except laser irradiation was to distinguish the In/CdTe/Au diodes with a Schottky contact (In/CdTe rectifying contact) and with a p–n junction and compare their characteristics. Both types of the diodes, i.e. fabricated without laser processing and by backside laser irradiation, showed quite similar electrical and spectroscopic characteristics. 6

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Fig. 5. Room temperature energy spectra of a 57 Co isotope taken with the In/CdTe/Au diode detectors fabricated without laser irradiation (Schottky diodes) (a) and with the backside laser doping technique (p–n junction diodes) (b) before (curves 1) and after annealing at 𝑇 = 200 ◦ C (curves 2). Reverse bias voltage was 200 V, except in (a), curve 2, when 50 V was applied.

(either with a Schottky barrier or electrical junction) [1,2]. The polarization effect is attributed to the non-uniform electric field due to the accumulation of charge carriers which generates a negative space charge near the anode contact after applying bias voltage [5,8]. Our previous studies revealed the similar behaviors of the electrical and spectral characteristics in both industrial (Acrorad’s) In/CdTe/Pt Schottky diodes and In/CdTe/Au diodes with a p–n junction formed by frontside laser irradiation doping [17]. However, the p–n junction diode detectors were slightly less subject to degradation with time.

Therefore, the research task was to find an evidence of laser-induced doping and creation of diodes with a p–n junction, not only with a Schottky barrier. The fact that laser irradiation of the In/CdTe interface, carried out from the CdTe side, shifts the forward branch of the I–V characteristic to lower voltages (i,e. forward current increases) and reduces reverse dark current of the In/CdTe/Au diodes (Fig. 3, curves 2–5) can be associated with the creation of a high-barrier p–n junction in the pCdTe crystals as result of laser-induced formation of a thin heavily In-doped n-type semiconductor layer. This reason of laser-stimulated modification of the electrical properties of the In/CdTe structures was supposed and discussed in the previous investigations [24,27,28]. However, the same or similar behavior of the I–V characteristics (an increase in the rectification ratio) could be due to changes in the Schottky barrier height as result of laser action on the metal–semiconductor interface [20,21]. Appropriate surface processing of semiconductors modifies the band bending at the crystal surface or at the metal–semiconductor interface, hence it changes the potential barrier height of the electrical contact. As known, the density of surface states significantly affects the bending of energy bands of semiconductors [35,36]. If this value increases and becomes high enough, the Schottky barrier height rises independently on the metal work function [35,36]. At large band bending, strong rectifying characteristics of a Schottky contact can be obtained. Therefore, the clarification of the reason of increasing the rectifying properties of the In/CdTe/Au diode structures after backside laser irradiation of the metal–semiconductor interface is a challenge task. The question is: have we obtained a p–n junction diode due to laser-induced doping of the thin semiconductor region or we have formed just a Schottky diode with a higher potential barrier due to laser action on the In/CdTe interface? Some attempts to prove the formation of an In-doped layer in the In/CdTe structures formed by frontside and backside laser irradiation were undertaken using the synchrotron radiation X-ray photoelectron spectroscopic study [30,31]. However, convincing proofs of the laserassisted creation of In/CdTe/Au p–n junction diodes have not been obtained and there is no cogent evidence that the formed In/CdTe/Au barrier structures are not just diodes with a laser-modified Schottky contact. It is known that, CdTe-based diode-type detectors suffer from the time instability of their electrical and spectroscopic parameters under bias voltage [5–8,17]. This phenomenon, generally termed as polarization, is the major draw-back of both types of CdTe diode structures

5.2. Thermal effect on the properties of In/CdTe/Au Schottky diodes By applying thermal annealing, we have shown for the first time significant differences in the properties of two types of diode structures developed using identical CdTe crystals, the same surface treatment, the same contact metals (In and Au) and electrode deposition technique, thus the same technological fabrication procedures except laser irradiation. The most noticeable differences were observed in In/CdTe/Au diodes heated up to 200 ◦ C (Fig. 4 and Fig. 5, curves 5 and curves 2, respectively). A significant increase in reverse dark current (Fig. 4(a), curve 5) and completed degradation of the spectra (Fig. 5(b), curve 2) of the In/CdTe/Au samples formed without laser processing are associated with deterioration of the In/CdTe Schottky contact and, therefore, degradation of the signal-to-noise ratio. As mentioned, a Schottky barrier is determined by the surface states of a semiconductor crystal and quality of the metal deposition [35,36]. It is expected and understandable that melting of the In contact drastically changes the interface states at the In/CdTe contact. The specific state, which was created to form a high Schottky barrier, is destroyed, and therefore the melted and solidified In contact loses its rectifying ability. In particular, reverse dark current in the In/CdTe/Au Schottky diodes annealed at 𝑇 = 200 ◦ C is much higher than in the samples before heating (curve 1) or annealed at temperatures lower than the In melting point 𝑇In (curves 2–4) (Fig. 4(a)). A marked deterioration of the performance of CdTe Schottky diodes after thermal annealing was observed in other investigations and this was attributed to significant changes in the electrical properties (carrier concentration, barrier height and ideality factor, depletion width, etc.) as result of modification of the interface state density and its distribution as well as carrier transport mechanism [37]. The results were discussed in terms of various theories of Schottky barrier formation [35–37]. Most likely, a lowering of the Schottky barrier height is due to 7

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layer, a p–n is formed at some depth which is equal of the thickness of the In-doped n-CdTe layer [26]. Thus, an electrical junction with a potential barrier is formed not at the In/CdTe interface as in the case of laser-unirradiated In/CdTe/Au diode structures (Schottky diodes), but in the underlying semiconductor region, i.e. inside the crystal at the boundary of the n-type CdTe:In layer and p-type bulk CdTe. The thickness of the In-doped n-CdTe layer has been estimated as 40–70 nm. Thus, the diode, created by the backside laser doping technique, is the multilayered structure: In-electrode (200 nm), In/n-CdTe ohmic contact, low-resistance highly doped n-CdTe:In layer (∼50 nm), abrupt built-in p–n junction, bulk part of semi-insulating p-like CdTe (0.75 mm), quasi-ohmic Au contact, Au electrode (200 nm). We suppose that the value of laser energy density determines the thickness of the doped n-CdTe:In region, i.e. the depth of the p–n junction formation, while the number of laser pulses mainly effects the concentration of dopant (In atoms), i.e. defines the sharpness of a p–n junction and its barrier height [25,26]. It is the fact that the electrical junction is located inside the semiconductor at a certain depth explains the absence of thermal deterioration of the electrical and spectroscopic properties of the In/CdTe/Au diodes, formed by backside laser irradiation, and their tolerance to annealing at temperatures below and even above the In melting point (Figs. 4(b) and 5(b)). Obviously, melting and solidification of the deposited In film (electrode) in the In/CdTe/Au p–n junction diodes does not affect the electrical contact between the metal and low-resistance semiconductor layer, which remains ohmic. It is hardly to expect that such relatively low-temperature (T = 150–200 ◦ C) annealing during rather short time (60 min) can significantly affect a shallow built-in p–n junction formed by backside laser irradiation doping, therefore the characteristics of the In/CdTe/Au p–n junction diode detectors are not significantly changed (Figs. 4(b) and 5(b)) with respect to Schottky-diode detectors with a surface barrier which suffers from such annealing (Figs. 4(a) and 5(a)). A slight decrease in total reverse dark current of the In/CdTe/Au p–n junction diode detectors, subjected to thermal annealing, can be due to a few reasons:

the diffusion of metal atoms of the rectifying contact into the interfacial layer [38]. Increased reverse dark current and other thermal-induced degradation effects in the In/CdTe Schottky contact (lowering the barrier height, increased noise, etc.) are significantly affect the signal-to-noise ratio of the annealed In/CdTe/Au Schottky-diode detector. The Schottky detector, subjected to heating at T > 𝑇In , cannot record the peak in the 57 Co isotope emission spectra, i.e. such detector almost completely loses its detection ability because of loss of events (Fig. 5(a), curve 2). The main reason of the deterioration of the In/CdTe Schottky contact properties is due to the fact that such type of contacts is formed just at the semiconductor surface as a metal–semiconductor interface and can be easily damaged by mechanical or thermal effects [35,36]. An irreversible deterioration of Cd(Zn)Te-based Schottky-diode detectors after thermal annealing at T > 𝑇In has earlier been reported, however the nature and contribution of the processes responsible for increased reverse dark currents and spectrometric degradation are not completely elucidated [37–42]. Moreover, low-temperature annealing of Schottky contacts (In(Au)/Cd(Zn)Te) in ambition air or vacuum leads to rather different results; in the first case, it may enhance the detector performance (lowering leakage current, increasing the detection and energy resolution), in the second one, it increases leakage current and detector performance can be worse [39,40]. In our case, vacuum annealing of the In/CdTe/Au Schottky diodes at 𝑇 = 200 ◦ C affects both the rectifying contact (interfacial layer) and near-surface CdTe crystal region, but not the bulk properties of the semiconductor because of not high enough temperature and relatively short time of the thermal treatment. However, the penetration of In atoms in the interfacial layer or even into the CdTe near-surface region can occur due to thermal diffusion and mainly because of dissolution of the semiconductor in the melted and heated In layer [30,31]. As result, a number of deep level defects in the CdTe near-surface region increases that is accompanied by an irreversible deterioration of the detectors [38]. Removing the In contact and CdTe surface layer by polishing regains the semiconductor properties and newly deposed In contact provides initially high Schottky diode characteristics. Thermal annealing of the In/CdTe/Au Schottky diodes at lower temperature does not cause so drastic deterioration of their properties. Heating to T < 𝑇In slightly and gradually worsens the electrical and spectroscopic characteristics. As seen, reverse dark current increases in 1.2–2.1 times at the annealing temperatures T = 50–150 ◦ C (Fig. 4(a), curves 1–4) The similar effects (a slight increase in leakage current and some deterioration of the detection properties) were observed in In/CdZnTe Schottky-diode detectors annealed in vacuum at 𝑇 = 130 ◦ C that was attributed to an increase in the number of cadmium vacancies due to cadmium sublimation [39]. As shown, evaporation of indium could be also one of the reasons for deterioration of the detector performance after annealing of the Schottky detectors in vacuum even at T < 𝑇In [39].

(a) modification of the p–n junction potential barrier including its height rise because of an increase in donor concentration in the n-CdTe:In layer, in particular at the p–n interfacial boundary as result of mass transfer of In due to the concentration diffusion of In and transfer of interstitial In atoms due to the thermal fluctuation jumps under the action of the driving force of the thermoelastic stresses and stress gradient [26]; (b) lowering the surface leakage current because of formation of oxides (TeO2 and others) on the lateral faces of the CdTe crystal and increasing oxide film thicknesses [40–42]; (c) changing the state of the quasi-ohmic Au/CdTe contact which is actually a low barrier Schottky contact, i.e. enhancing the Au/CdTe interface, increasing the barrier height [39–41], or in contrary, improving ohmic properties of the Au/CdTe contact due to thermal diffusion of Au atoms which replace Cd sites or occupy Cd vacancies as acceptors [42]; (d) decreasing the bulk current due to thermal-induced transformation of the point defect structure of the CdTe crystals [1,32, 33].

5.3. Thermal effect on the properties of In/CdTe/Au p–n junction diodes In contrast to the Schottky diodes described above, thermal annealing of In/CdTe/Au diode detectors, fabricated by the backside laser doping technique, carried out in a wide range of temperatures (below and above the In melting point) did not deteriorate their performance. Moreover, one of the key characteristics of diode detectors, i.e. reverse dark current, decreased after annealing even in the case of melting and solidifying the In contact (at 𝑇 = 200 ◦ C) (Fig. 4(b)). This feature along with the spectroscopic characteristics, which almost did not change after the annealing (Fig. 5(b), curve 2) has allowed us to consider that backside laser irradiation of the In/CdTe interface transforms In/CdTe/Au Schottky diodes into p–n junction diodes. Certainly, if direct laser irradiation of the In/CdTe interface results in doping of the surface region of the p-CdTe crystal (near the interface) with In atoms (donors) and thus, creates a thin n-type semiconductor

The mentioned processes, occurring at low-temperature annealing of CdTe-based detectors can enhance their performance, in particular improving the isotope spectrum profile (position, height and symmetry of the peak, energy resolution, etc.) [39,40]. In our case, the symmetry of the 122 keV peak in the 57 Co isotope emission spectrum taken with the In/CdTe/Au p–n junction diode detector after annealing at 𝑇 = 200 ◦ C is higher (Fig. 5(b), curve 2). This evidences a decrease in the hole tailing effects [40]. A little decrease in the 122 keV peak height can be due to a slight blurring of the p–n interfacial boundary and thus, 8

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some depth, remains functional even after melting and solidifying the In electrode. Certainly, Al electrode, used as a rectifying contact in Al/CdTe/Pt diode detectors, is not melted at such temperatures [6–8]. However, deposition (thermal evaporation in vacuum) of Al is more difficult than In and, anyway, heating of Schottky diodes negatively affects their characteristics [7,8]. The characteristics of the In/CdTe/Au p–n junction diodes remained almost unchanged or changed just a little at thermal annealing, therefore such diode detectors are promising for creation of X/𝛾-ray pixel sensors when thermal bonding procedures are important. The backside laser doping technique will be optimized to create a p–n junction with a higher barrier. This allows suppress leakage current and apply higher bias voltage to such diodes. An increase in the electric field, generated by the external bias, compared with the internal field due to the charge accumulation, opens more possibilities to reduce the polarization effect and improve the stability of CdTe p–n junction diode detectors during long-term operation.

reducing the sharpness of the p–n junction as result of the thermal diffusion of In. Certainly, the processes (b)–(d) should decrease reverse current also in the case of the In/CdTe/Au diodes, fabricated, without laser irradiation, however the thermal degradation of the In/CdTe Schottky contact plays the dominant role and leads to an increase in the bulk leakage current and deterioration of the electrical and spectroscopic characteristics, in particular after annealing at 𝑇 = 200 ◦ C (Figs. 4(a) and 5(a)). The mechanisms of laser-induced doping have been briefly discussed for the cases of frontside [15–20,26] and backside laser irradiation of In/CdTe structures [27–31]. However, the elucidation of the roles of laser-stimulated processes and mechanisms of transformation of the point defect structure of CdTe crystals under backside laser irradiation doping requires additional studies. 6. Summary and conclusion

CRediT authorship contribution statement

The method of creation of p–n junction diode X/𝛾-ray detectors have been developed, based on backside laser irradiation of the In/CdTe structures, as an alternative to frontside laser irradiation [14–20]. Employing this technique allowed us to provide extreme conditions in the confined area at the semiconductor–metal interface for introducing high concentration In atoms acting as donor into the near-interface ptype semiconductor region and thus, to heavily dope a thin CdTe layer, form a shallow abrupt p–n junction and as result, to obtain efficient In/CdTe/Au diode-type detectors. Fabrication of the similar In/CdTe/Au structures without laser irradiation made it possible to carry out a comparative study of the p–n junction and Schottky diodes. Despite both types of the created diode detectors demonstrated the similar electrical and spectroscopic characteristics, low-temperature thermal annealing of the samples revealed a fundamental difference in the barrier formation at the In/CdTe interfaces. A key feature was the deterioration of the I–V characteristics (a significant increase in reverse dark current) and complete degradation of the 57 Co emission spectra for the samples, fabricated without laser processing and subjected to annealing at 𝑇 = 200 ◦ C, while the operating performance of diode detectors, formed by backside laser irradiation, was found to be enhanced at the same thermal treatment. Thus, it has been experimentally proved that backside laser irradiation of the In/p-CdTe structure results in doping of the nearinterface CdTe region with In atoms (donors), creates a thin n-CdTe layer and thus, forms a shallow abrupt p–n junction. The properties of the In/CdTe/Au diodes with a built-in p–n junction did not deteriorate after heating, melting and solidifying the In contact in contrary with the In/CdTe/Au Schottky diodes which drastically degraded when the In rectifying contact was melted and then solidified. This was resulted from an increase in the level of electronic noise in the annealed Schottky diodes and, therefore, loss of events was due to the deterioration of the signal-to-noise ratio. Moreover, reverse dark current of the initial In/CdTe/Au p–n junction diodes was lower than that of the In/CdTe/Au Schottky diodes that was attributed to higher efficiency of the p–n junction potential barrier as well as a decrease in the bulk leakage current due to the effect of laser irradiation on the point defect structure of the CdTe crystal even the semiconductor is transparent for such radiation [32,33]. Annealing both types of In/CdTe/Au structures at temperatures lower than the In melting point affected leakage current in different ways: it slightly decreased in p–n junction diodes and increased in Schottky ones. The fact that the In/CdTe/Au diode detectors with a p–n junction formed by the backside laser irradiation technique show excellent tolerance to thermal treatment has great importance because makes it possible to employ bonding and bumping procedures using heating up to temperatures of the In contact melting or higher. At such procedures, a Schottky contact at the In/CdTe interface is degraded, while a p– n junction, created in the underlying region of the CdTe crystal at

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