IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 5, MAY 2013 695 Dosimetric Performance of Single-Crystal Diamond X-Ray Schottky Photodiodes Alessandro Bellucci, Stefano Orlando, Domenico Caputo, Emilia Cappelli, and Daniele M. Trucchi Abstract— Single-crystal diamond Schottky photodiodes have been developed following a WC–diamond–TiC/Ti/Ag vertical structure for X-ray dosimetry with characteristics of low leakage current and the capability of zero-bias operations. Continuous and modulated X-ray measurements show that the devices developed provide a linear response to dose rate at low bias voltages (< 2.5 V) and at modulation frequencies > 130 Hz. This supports their application in modulated radiotherapy treatments. Index Terms— Diamond, fast response, linearity, radiation dosimeter, Schottky diode. I. I NTRODUCTION D IAMOND is considered the most appropriate material for X-ray dosimetry, since it presents properties of tis- sue equivalence, radiation hardness, and chemical inertness [1], [2]. The latest developments of X-ray diamond dosime- ters aim at improving real-time measurements of the dose rate delivered to the patient during radiation therapy. In particular, intensity-modulated radiation therapy needs very fast dosimeters, capable of following the very small and inhomogeneous field of irradiation used during cancer treat- ments [3]. Single-crystal diamond, produced by CVD, seems to meet these requirements [4]. The commercial availabil- ity of single-crystal films is stimulating scientific inter- est in the development of new dosimeter structures, to reach the best solution for precise measurements of the radiation dose and dose rate [5]. A promising possibil- ity to optimize X-ray detection is represented by the development of a Schottky photodiode in transverse con- figuration, characterized by low leakage current, capabil- ity of zero-bias operation, and fast response time [6]–[8]. Such devices have been developed, and the results, in terms of dosimetric performance, are presented here. II. EXPERIMENTAL DETAILS Diamond samples used in this letter are electronic-grade high-purity single crystal films (size 4.0 × 4.0 × 0.5 mm) produced by Element Six Ltd. with (100)-oriented oxidized surfaces. Oxidation is obtained by means of a sample treatment at 200° C with a solution of H 2 SO 4 + HNO 3 + HClO 4 Manuscript received March 4, 2013; revised March 16, 2013; accepted March 16, 2013. Date of current version April 22, 2013. The review of this letter was arranged by Editor C. Jagadish. A. Bellucci, S. Orlando, E. Cappelli, and D. M. Trucchi are with CNR-IMIP, Monterotondo Scalo 00015, Italy (e-mail: alessandro.bellucci@imip.cnr.it). D. Caputo is with the Department of Electronic Engineering, La Sapienza University, Rome 00185, Italy. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2013.2253595 (1:1:1), in order to remove surface contaminations and to avoid shunt currents. The electrical resistivity is > 10 14  cm. Rectangular-shaped electric contacts (3.8 × 3.8 mm) were fabricated on the sides of the diamond film by pulsed laser deposition (PLD). The single-crystal diamond film, which is the active material, is sandwiched between a tungsten carbide (WC, 150-nm-thick) Schottky contact and an ohmic contact, consisting of a multilayer of titanium and silver (50- and 100-nm-thick, respectively). Thermal annealing induces the formation of a very thin layer (few nanometers) of titanium carbide (TiC) on the diamond surface; this layer is responsible for the pure ohmic behavior of the contact, since it acts as a tunneling layer for the passage of charge carriers even if the semiconductor is not doped [9]. An Ag layer (50-nm- thick) is deposited as a protection coating to avoid oxidation of the underlying Ti layer. After deposition of electric contacts, the devices were annealed at 450° C for 1 h under Ar atmosphere (40 torr) to achieve good ohmic properties. The use of WC is related to its properties of thermal and radiation resilience [10]. WC–diamond junction keeps its rectifying behavior, even if it is processed under a high-temperature annealing. The configuration is then a vertical WC–diamond– TiC/Ti/Ag structure, where WC–diamond acts as a rectifying junction and diamond–TiC/Ti/Ag as an ohmic one. The test radiation source used is an Al-filtered X-ray copper tube ( K α line at 8.06 keV) with a fixed accelera- tion voltage of 40 kV, whose anodic current was changed to obtain variations of radiation dose rate. An ionization chamber (NE 2536) was used as a reference detector system to measure absolute dose rate; a previous calibration allowed the most suitable corrections to be applied to the ionization chamber response. Continuous-wave (DC) measurements were performed by interfacing the devices to a picoammeter/voltage source (Keithley 487), while modulated-wave (AC) measure- ments were performed by using a mechanical chopper for beam modulation up to a frequency of 550 Hz. An elec- tronic chain composed of a transimpedance amplifier (Signal Recovery 181) and a phase-sensitive lock-in (Signal Recovery 7265) has been employed to measure the dosimeter radiation monitoring system signal. III. EXPERIMENTAL RESULTS The characterization of the detectors were initially per- formed under DC irradiation by varying the dose rate (DR) from 0.016 to 0.860 Gy/h (minimum and maximum values of the Cu tube operating stability), and the bias voltage V bias values in the ± 75-V range. DC photocurrent-to-voltage characteristics under irradiation show the typical behavior of a 0741-3106/$31.00 © 2013 IEEE