**1. Introduction**

The concept of using compound semiconductors as radiation detectors was introduced in 1945 by Van Heerden [1,2], who was the first to be able to detect alpha and gamma rays with solid-state radiation counters. His pioneering results gave rise to a new class of radiation detectors, which is now commonly known as semiconductor detectors. Compared to gas detectors, semiconductor detectors require much lower average energies for the creation of electron-hole pairs (30 eV for gas [3], 3.7 eV for Si [4], 7.8 eV for 4H-SiC [5]), which bring higher energy resolution in radiation spectroscopy [6,7]. Since the 1960s, the most commonly used semiconductor materials have been high-purity silicon (Si) and germanium (Ge), the main limitation of which is that they must operate at liquid nitrogen temperature. Since the 1990s, intense research activity has been carried out on other semiconductors for manufacturing detectors able to operate at room temperature, such as gallium arsenide (GaAs), cadmium telluride (CdTe), and cadmium zinc telluride (CdZnTe) [8–12]. In the last two decades, silicon carbide (SiC) has obtained increasing interest in the field of radiation detectors due to the achievement of a high purity level in the crystal structure and considerable thickness (>100 μm) in the epitaxial layer. This finally achieved recognition for semiconductor detectors as a real alternative to Si-based radiation detectors, which present possibilities but also limitations at and above room temperature, as well as in high-radiation environments [13,14]. There are certain properties that make SiC especially suitable for the realization of ionizing radiation detectors. Thanks to the wide energy bandgap of the polytype 4H-SiC (3.26 eV), which is three times higher than that of Si (1.12 eV), electronic devices fabricated in such material can operate at extremely high temperatures without su ffering from negative e ffects, due to thermally generated charge carriers [15]. Silicon carbide radiation detectors benefit from this property because the wide energy bandgap allows the achievement of very low leakage currents, i.e., very low noise levels, even at the high electric fields applied during their operation. Moreover, the high thermal conductivity of 4H-SiC (3.8 W/cm◦C) enables SiC devices to dissipate large amounts of excess generated heat, which would cause a temperature increase, responsible for degradation of the device's performance. High thermal conductivity is useful for increasing the radiation hardness of the detector, as well as for controlling the operating temperature when the front-end electronics are close to, or in contact with, the detector [16]. Furthermore, SiC can withstand an internal electric field over eight to ten times greater than GaAs or Si (2 MV/cm for 4H-SiC vs. 0.4 MV/cm for GaAs or 0.3 MV/cm for Si) without undergoing avalanche breakdown. This property enables the fabrication of very high-voltage devices [17]. In the case of X-ray detection and spectroscopy, the high breakdown field of 4H-SiC allows, in principle, the detector to work always in the regime of saturated-electron and hole-drift velocities, independently of the detector's active region width. When this operation condition is coupled with epitaxial material of high crystalline quality, a full and fast charge collection can be expected [16], as well as a high sensitivity, as already demonstrated [18]. Such properties allow SiC-based devices to be operated without any costly, bulky, and power-consuming cooling systems, as in the case of Si- or Ge-based devices, while maintaining an excellent signal-to-noise ratio over a wide range of temperatures. This leads to notable advantages in terms of the lower cost, more compact size, lighter weight, lower power consumption, and higher performance of SiC detectors. Further explanation of the electrical properties of SiC in connection with the ionizing detector performance benefits can be found in [16].

Microstrip detectors find application where the position of the radiation interaction is necessary information for the physical process to be studied. The advantage of using microstrips with respect to other position-sensitive detectors, such as pixel detectors, is a lower number of readout channels. Several microstrip detectors have been developed in Si for high-energy physics, or in Ge, CdTe and GaAs for X-ray spectroscopy [19–22]. In this work, we investigated the electrical and spectroscopic performance of two innovative position-sensitive radiation detectors in epitaxial 4H-SiC, using microstrip geometry. The detectors were characterized in detail at di fferent temperatures and applied bias voltages. The obtained results are presented and discussed in the following sections.

#### **2. Materials and Methods**

Two di fferent designs of silicon carbide microstrip detectors have been realized on top of two-inch high-purity epitaxial 4H-SiC wafer produced by LPE Epitaxial Technology Center [23]. Each detector consists of 32 strips with a length of 2 mm, a width of either 25 μm (SM1) or 50 μm (SM3), and a pitch of either 55 μm (SM1) or 100 μm (SM3)—see Figure 1. Each of these strips can be read out independently by a front-end electronics channel, and therefore behaves as a separate detector. A cross-sectional view of the 4H-SiC microstrip structure is shown in Figure 2. The SiC epitaxial layer, which is the active region of the detector, has a maximum thickness of 124 μm, as experimentally measured (Figure 3).

The application of a reverse bias at the common back electrode (ohmic contact) creates a depletion region, *xd*, depending on the applied bias, *VR*, and the residual doping (donor) concentration, ND, within the material, according to

$$\mathbf{x}\_d = \sqrt{\frac{2\varepsilon\_0 \varepsilon\_r}{qN\_D} \left(\psi\_{bi} - V\_R - \frac{kT}{q}\right)}\tag{1}$$

where ψ*bi* is the built-in potential and the term *kT q* arises from the contribution of the majority-carrier distribution tail [24].

**Figure 1.** Photographs of the two microstrip detectors used in this work, together with a detail of their peripheral regions with bonding pads.

**Figure 2.** Cross-sectional view of the 4H-SiC microstrip structure.

The residual doping concentration depends on the homogeneity of the epitaxial layer. The donor concentration profile, *ND*(*x*), was determined as a function of the depleted layer width from capacitance-voltage measurements, as described in Section 3.
