1. Introduction
Single crystals are highly established in the vital areas like radiation detection, solid state lasers, optical windows etc., and are also widely used in emerging areas like nano technology for the synthesis of thin films and quantum dots in which the substrates used are invariably made from single crystals. There has been a higher interest in the study of alkali halide single crystals due to expanding possibilities of their practical applications [1-2]. The optical properties of alkali halide in pure and doped with rare earth materials are the main focus due to its enhanced applications in dosimetry, solid state lasers and scintillation applications. The use of alkali halide single crystals does not give us much room for quality improvement in certain domains and applications due to lack of crystalline perfections. The perfection of the grown crystal and doping with suitable elements are important aspects which influences the physical properties and optical characteristics [3]. Therefore, efforts are continuously being made for improving the techniques used to grow defect free crystals. As per the literature, nearly 80% of all the single crystals are grown by melt growth techniques due to relatively faster growth rates and suitability for large scale production. Out of the many melt growth techniques, Czochralski and Bridgman techniques are much popular and are used to grow most of the halide and oxide single crystals for material studies as well as for the industrial production. The Czochralski (Cz) technique is a popular method of crystal growth because it can produce large, dislocation free crystals at a relatively faster rate. Vibration free slow translation and rotation movements are mandatory to produce good quality dislocation free crystals and many modifications and automations have been carried out by researchers in Bridgman and Czochralski instrumentation over a period of time [4]. Hence, to improve the quality of crystals, a stepper motor based remotely operated Czochralski set-up, Automatic Crystal Puller System (ACPS), was developed. The system consists of mechanical translation and rotation set up carefully coupled to respective translation and rotation stepper motors operating in the micro stepping mode. Programmable System-on-chip (PSoC) based controller and a firmware tailored to suit the crystal growth applications was developed. This work concentrates on the development of a nano resolution Czochralski system, growth of alkali halide crystals using the system and optical and dosimetric properties of rare-earth ions in alkali halides. In this work, single doping of Eu and Ce in KCl matrix and the effect of Ce co doping in KCl:Eu crystals were studied. Eu 2+, Sm2+ and Yb2+ are the most-studied divalent rare-earth species in this category but very few studies focussed on the dosimetric and scintillation applications of alkali halide crystals [5-6]. In this study, TL and OSL characteristics of Eu and Ce doped KCl single crystals and the effect of double dopants in the base KCl matrix were analysed. An intense OSL emission of 420 nm was reported for X-ray irradiated KCl: Eu by Nanto et.al [7] which paved the way for further investigation on the rare earth doped KCl crystals. The detailed design and development, salient features and advantages of the system along with the optical properties of the grown rare earth doped alkali halide single crystals with a prime focus on the dosimetric applications are depicted in this paper.
2. Design description of ACPS system
Crystal growth by Czochralski method is based on slow pull of seed from melt free surface. In this method, with changing of pulling speed of seed and rate of increasing and the decreasing of the furnace temperature, would result a crystal with needed size and shape [8]. In order to achieve vibration-free, uniform feed movements apart from slow translation rate, a high precision translation and rotation set up was developed. A Cypress programmable device (PSoC) based controller manages all the operations of the ACPS with the help of the firmware tailored to suit the requirements. Modular development is followed for easy maintenance of the system.
2.1 Hardware design
The block diagram of the ACPS instrumentation design is shown in Fig 1.
The design of translation and rotation setup with commonly available Direct Current (DC) motors have many drawbacks like difficulty in motion control, position control etc. This leads to the incorporation of more sensors for the purpose which will complicate the electronics of the system. Stepper motors are used where high precision and accuracy is of major concern. Stepper motors available in the market are not really designed for smooth rotation as it rotates with a basic step of 1.8o, but they are designed to make position and speed control very easy. Most of the commercially available stepper motors take 200 step positions per revolution. Hence, on direct connection of stepper motor output shaft to a translation setup, the stepper will ‘jump’ from one position to the next where each position is 1/200th (1.8°) of a complete rotation[4]. This motion results in undesired jerk and vibration during the crystal growth which affects the quality of the crystals. Smoother movements are essential for better quality crystals especially for growing crystals for dosimetric and scintillation applications using Bridgeman and Czochralski method. The position resolution and smoothness of conventional stepper motor can be increased by micro stepping mode. Hence, micro stepping mode operation of the motor is desirable for achieving smooth, vibration-free, and precise translation applications.
2.2. Implementation of micro stepping mode of operation
In micro stepping mode, instead of switching the current in a winding from ‘ON’ to ‘OFF’, the current is scaled up and down in smaller steps. When two phases are turned on and the current on each phase is not equal, the rotor position is determined by the current phase ratio. This changing current ratio creates discrete steps in the torque exerted on the rotor and results in smaller fractional steps of rotation between each full step [9]. A drive mechanism called ‘chopper drive’ is used to implement micro steps by varying the current through the motor winding sinusoidally with a 90o phase shift. The total torque exerted is the vector addition of the torque exerted on the rotor due to the two windings. Each of the torques is proportional to the position of the rotor and the sine/cosine of the step angle. Since the torque is proportional to the current in the windings, by varying the current, the position of the rotor can be controlled.
The PSoC 3 (CY8C3866AXI) architecture consists of digital subsystem which provides unique configurability of functions and interconnects. The stepper motor control uses these digital resources to implement timers, control registers, system bus, flash memory and clocking system to implement micro stepping [9]. A timer register generates periodic clock pulses that is used to generate each step (or micro step) of the motor according to the requirement. This timer can be used to run the motor at a specific speed, or to a specific position (exact number of steps). To set the speed of the motor, timer is updated by firmware when the set value was received from the remote desktop device, say a laptop. The chopper drive has built-in microcontroller and receives clock pulses from PSoC controller. These clock pulses corresponding to the motor speed parameters as set by the user, would be converted into corresponding PWM outputs and applied to the Metal-Oxide-Field-Effect Transistor (MOSFET) driver (2D M542 Leadshine Technology)[10]. The MOSFET drive controls the current output in the stepper motor windings to achieve micro stepping. Micro stepping is achieved by dividing the basic step of 1.8° by 25000 pulses using the firmware and further reduction of two times by mechanical reduction gear. Hence, one complete rotation of the translation motor is achieved through 50000 effective steps. The linear translation is accomplished by coupling the rotating gear of the stepper motor to translation set up which has a moving rod with pitch of 1 mm, giving a resolution of 20 nm linear translation for one micro step. For this purpose, a translation set up has been manufactured with high precision Computer Numerical Control (CNC) machines. With this ultra-smooth and uniform speed control, vibration-free movement can be realized. Digital output port is used to drive the character Liquid Crystal Display (LCD) of the system which displays the real time status of the ACPS system, like mode of operation of the system, rotation and translation mode and speed, direction of motion of motors, total run time for the crystal growth etc. at the device end. Photograph of the ACPS system is shown in Fig. 2.
2.2 Software design
The application software was developed in two main categories, Firmware and Graphical User Interface. The firmware was developed using “PSoC creator†IDE, which executes the micro stepping algorithm based on the input values from the remote desktop device. Graphical User Interface developed in LabVIEW which helps the user to input the growth parameters to the ACPS.
2.2.1. Graphical User Interface (GUI)
A user friendly GUI was developed using LabVIEW software which runs in the remote desktop device. The developed GUI act as a bridge between the ACPS and the user and it will help the user to control the system without entering to the furnace room. This also helps the user to input parameters for crystal growth in a simple and effective way.
ACPS has two mode of operation, Manual and Auto. Manual mode is mainly used at higher translation rates and will be useful for positioning purpose, i.e., for inserting the seed crystal into the melt and taking out the grown crystal from the furnace. In this mode rotational mode is not enabled. In the auto mode, both translation and rotation motor direction and speed are programmable. The translation motion can be programmed from 0.001 mm/hr to 99.99 mm/hr and rotation motion from 0.001 rpm to 99.99 rpm. The direction of translation and rotation motors and mode of system operation are selected using a toggle switch and indicated by green LED in the GUI. All the required parameters for crystal growth viz., total run time, translation and rotation movement, rotation profile, are send to the ACPS system using Radio Frequency (RF) mode. The ACPS receives all the parameters from the external desktop device and calculates the number of clock pulses to be inputted to the stepper motor drives. Apart from the normal rotation mode, ACPS is provided with ‘Wave Profile Mode’ of rotation operation, which might be required for advanced crystal growth applications. In this mode, the rotation motor can be programmed such a way that the rotation rate can be reversed and accelerated/decelerated in equal intervals of time. Hence, the GUI presents wide options for a user to choose from for growing defect free crystals.
2.2.2. ACPS Firmware
The firmware, tailored to suit the requirement was created using PSoC creator†IDE. The firmware was burned in the PSoC Read Only Memory (ROM) that controls and manages the operations of the ACPS. The firmware receives and decodes the commands and by changing the pulse sequence applied to the windings of the motor drive to get the required translation and rotation rates and mode of operation.
2.3. Wireless connectivity
The RF connectivity between the ACPS and desktop device is established using Zig bee modules. ZigBee modules were attached to the PSoC board through Integrated Drive Electronic IDE) interface and to the desktop device through USB interface. The GUI is so developed that the desktop top device can be dis-engaged from the task once the ACPS commands are given, i.e. there is no need to have continuous connectivity for the ACPS system to operate. The device can be linked back to the ACPS to know the status of the system by clicking ‘Refresh’ button in the GUI.
3. Application of the ACPS for the growth of KCl crystals
Single crystals of KCl have always been a researchers favourite due to its high band gap (6-10 ev) and many properties of KCl with and without dopants have been reported by many researchers . Intense research has been continuing to explore the characteristics of the crystals in diverse areas like optical window materials, scintillation and radiation detections etc. Developments over the past two or three decades in OSL and TL dosimetry have led to the application of these crystals in many of the radiation dosimetry fields, including personal, environmental, retrospective, space, neutron and medical dosimetry[11-12]. In the present work KCl:Eu, KCl:Ce, KCl:Eu ,Ce single crystals were grown using ACPS by Czochralski technique and optical characterization were carried out to find out the feasibility of their use in the area of radiation dosimetry.
3.1 Experimental procedure
The powders for preparing various rare earth doped single crystals were KCl (99.99%, Aldrich) and EuCl3.6H2O (99.9%, Aldrich) CeCl3.7H2O (99.9%, Aldrich). The concentration of Eu and Ce in the KCl melt was 0.2 mol%. All the chemicals were carefully weighed and mixed thoroughly in alumina crucible and placed in the furnace having a temperature controller (accuracy is ±1 ◦C) and heated till the whole substance was melted. The temperature was set slightly above the melting point to ensure the homogeneous molten state. Single crystalline seeds of pure KCl were used. The pulling rate was maintained nearly 2-4 mm per hour and a rotation rate of 4–6 rpm. In all cases the single crystalline seed crystals were oriented along the [1 0 0] axis. Finally the furnace was allowed to cool to room temperature at the rate of 1oC per minute in order to avoid the cracks due to thermal shocks in crystals. The Photograph of the grown crystals are shown in Fig. 3(a), Fig. 3(b), Fig. 3(c), respectively.
XRD of all the samples in the powder form were recorded using X ray diffractometer (GNR Explorer, Italy). The PL spectra were recorded using a Jobin Yvon-Spex Spectro-fluorometer (Fluorolog version-3; Model FL3-11) at room temperature. The excitation source used was a Xenon arc lamp (450 W). The detector (PMT-R928P) has flat response from 200 to 900 nm. The TL glow curves and OSL spectra were recorded using TL/OSL reader supplied by Nucleonix India Pvt. Ltd.
4. Results and discussions
4.1 Structural properties
Crystallographic structure was determined using the XRD studies. Fig. 4 shows X-ray diffraction pattern of powdered samples of KCl:Eu, KCl:Ce and KCl:Eu,Ce single crystals.The XRD of the crystals were compared with ICDD patterns (00-041-1476) and diffraction peaks can be assigned to KCl with the cubic crystal structure. However, a small shift in the diffraction peaks towards lower angle for Eu 2+ and Ce3+ doped KCl was observed compared to undoped sample. This confirms the expansion of the KCl lattice and existence of dopants certainly in the KCl matrix. Since the ionic radii of K+ (ri = 0.138 nm) is higher than that of Eu 2+and Ce3+ ion radii ( ri = 0.131, 0.101 nm respectively) a contraction of the lattice and shift towards higher angle side was expected. However, we observed the shift in the XRD peaks towards the lower angle side. A similar lattice expansion have also been observed in Ce3+ doped NaCl and Ce3+ doped KCl where the ion radius of Na+ (ri = 0.102 nm) and K+ are larger than that of Ce3+ [13-14]. This might be due to the fact that in case of divalent and trivalent substitution unlike monovalent atoms; more complex defects are formed satisfying the charge compensation [15] and also might be due to the presence of dopants in the interstitials.
4.2 Photoluminescence studies
Photoluminescence studies of KCl:Eu, KCl:Ce and KCl: Eu, Ce were carried out to find out the emission characteristics of crystals. PL studies of KCl:Eu, KCl:Ce phoshors and Ce and Tb doped glass matrices and have been reported earlier[ 16,17] . Fig. 5 (a) shows the PL spectrum of Ce co-doped KCl:Eu at Room Temperature (RT). KCl: Eu,Ce showed good PL intensity peaking at 421 nm which is the characteristics of the Eu2+ emission when excited at 280 nm. This showed that the crystal could give good PL emission possibly due to the coincidence of Ce3+ emission and Eu2+ excitation.
The energy level scheme of the KCl :Eu, Ce with optical transitions and energy transfer process is shown in Fig 5 (b). As Ce3+ excitation energy is in 250-350 band, on excitation of 280 nm, electrons are pumped to 5d level and then relaxes non radiatively to the lowest component of 5d level. It is seen that there is an overlapping of emission of co activator Ce3+and excitation spectrum of Eu2+. As the value of excited 5d state of Ce 3+ is close to the 4f65d1 levels Eu 2+ ions, it is highly possible that energy transfers from Ce3+ to Eu 2+ ions occur and giving rise to an intense PL emission due to the transition of 4f65d1 to ground state 4f7 (8S7/2) of Eu 2+ [18].
4.3. Thermally Stimulated Luminescence (TSL) studies
The introduction of impurities is important for activation of phosphors and for the optimization of radiation dosimeters since it creates defects in the crystal lattice which can create trapping centers. The present investigation concerns the effect of different impurities on the TSL of KCl crystals. TSL glow curves for KCl crystals incorporating various impurities are shown in Fig. 6. The TSL of the samples were taken after gamma irradiation with Co-60 source. Glow curves for KCl crystals containing different concentrations of Eu as an impurity has been already studied and observed that the presence of Eu2+ will enhance the TL emission [19]. The samples were read using TL reader with a heating rate of 5oC/Sec. KCl : Eu2+ showed very low intensity TSL peak at ~225oC and the same has been observed by many researchers [20]. At the same time, KCl:Ce gives good TL peak around 230oC and a small low temperature peak at ~100oC. Co-doping with Ce in KCl: Eu showed that low temperature peak of KCl:Ce was suppressed and only glow peak at 230oC became prominent with a slight intensity enhancement. This study showed that that co-doping of Ce in KCl:Eu would result in a good TSL material compared to single doping as it suppresses the low temperature peak and enhances the intensity slightly. An ideal TL dosimeter should have a peak around 200-250oC since a low temperature peak can result fading of signals during storage.
4.4. Optically Stimulated Luminescence (OSL) studies
Optically stimulated luminescence (OSL) is the luminescence emitted by crystalline insulators and semiconductors that were previously irradiated on stimulation by light. This transient signal is produced due to the release and recombination of charges trapped in defects in the material, which was created by exposing the material to ionizing radiation [21]. OSL is gaining momentum in personal dosimetry use due to the availability of high sensitivity detector materials, re-readability of the signal. The physical process underlying the OSL phenomenon is similar to that occurring in TL dosimeters, with the exception that light is used to stimulate the signal instead of heat[22]. OSL spectrum of KCl:Eu, KCl:Ce and KCl:Eu,Ce single crystals were taken after irradiating with Co-60 source and is shown in Fig. 7. Good OSL intensity emission from KCl: Eu and KCl:Ce was observed. Also it is found that by co doping with Ce in KCl:Eu, the OSL intensity increases by two fold. As TSL studies also showed similar trend which underline the fact that alkali halides with suitable rare earth combination can work as a good dosimeter.
The emission of Eu2+ is strongly dependent on the type of host lattice and hence the choice of host is critical in determining the optical properties of Eu2+ ions. In order to increase the emission of Eu2+ ions within the spectral range, a co-activator that has strong absorption in this spectral range and transfers its energy to Eu2+ can be chosen and this is another effective way of improving the photoluminescence properties of phosphors. Energy transfer from a co-activator Ce3+ to an activator Eu2+ has been reported for several hosts (23-25). Co-doping of Ce3+ can enhance the emission of Eu2+ due to energy transfer from Ce3+ to Eu2+, and Ce3+ plays a role as a sensitizer. Hence, observed TSL and OSL intensity in KCl: Eu co-doped with Ce might be due to the possible energy transfer from Ce 3+ to Eu2+ . The increase in intensity in TSL and OSL showed that KCl:Eu, Ce exhibit good optical properties after gamma irradiation and suitable candidate for the application in radiation dosimetry. However, more studies to be carried out to see whether the intensity can be further enhanced by varying the dopant concentrations
5. Conclusions
A stepper motor based remotely operated Automatic Crystal Puller System (ACPS) was developed for crystal growth applications using Czochralski technique. The system consists of mechanical translation and rotation assembly coupled to respective translation and rotation stepper motors operating in a micro stepping mode. Cypress programmable device, PSoC, based hardware is used as the controller and firmware was developed in ‘PSoC creator IDE “to control all the operations of the system. The system has a programmable bidirectional rotation speed of 0.001 rpm to 99.99 rpm and translational motion as low as 0.001 mm/hr to 99.99 mm/hr. Software controlled pulse based step reduction was effectively implemented for smooth movements. Linear translation resolution of 20 nm was achieved using the system. LabVIEW based user friendly GUI and the firmware enables remote operation of the ACPS from a remote desktop device without entering to the furnace room. Rare earth doped alkali halide single crystals, KCl:Eu, KCl:Ce, KCl:Eu,Ce were grown using the system and optical characterisation were carried out. The TSL and OSL spectra of the grown crystals showed that co doping with Ce in KCl:Eu leads to enhanced optical properties compared single doping, though optimisation of impurity concentrations only can give the exact picture of high luminescence yield. These results indicate KCl:Eu, Ce can be potentially used as TL and OSL dosimeter due to the high sensitivity when exposed to ionizing radiation.