Particle physics experiments

How crystal growth experiments will help in the search for ghost particles

Deciphering the origin of the Universe through the search for dark matter ghost particles and Majorana neutrinos requires next-generation crystal growth experiments to further reduce their background from cosmogenic induced radioisotopes.

The Big Bang theory, which predicts matter-antimatter symmetry in the nascent universe, is widely accepted to explain the origin and early evolution of the universe. However, observational evidence indicates that the structure of the Universe today is dominated by matter. Therefore, the matter-antimatter symmetry was broken by an unknown physical process. It is an irresistible mystery. Additionally, experimental evidence suggests that 80% of matter is dark matter, which does not glow or reflect light. Despite great efforts so far, dark matter continues to have a ghostly nature. Germanium (Ge)-based experiments have the lowest detection energy threshold in the search for dark matter among large-scale experiments.

Majorana neutrinos (neutrino and antineutrino are the same particle) could shed light on the physical process that broke the matter-antimatter symmetry, if proven to exist. The only practical way to prove that neutrinos are Majorana particles is to detect double beta decay without neutrinos. This is a non-standard form of nuclear decay and only a few nuclei can undergo this decay process. 76Ge-based detectors have the best energy resolution to uncover this rare decay process. Therefore, Ge-based experiments are essential in the search for dark matter ghost particles and Majorana neutrinos. However, cosmogenically produced long-lived radioisotopes can undermine the detection objective and hamper the sensitivity of discovery.

The growth of crystals underground and the development of detectors can greatly reduce cosmogenic processes and thus maximize the potential for discovery in the detection of dark matter and proof of the existence of Majorana neutrinos. This means that the purification of Ge, the growth of Ge crystals and the fabrication of the Ge detector must be implemented in an underground environment where the experiments will be built.

Ge material

Ge is a rare element in the earth’s crust. The abundance of Ge is estimated at around 7 ppm. Ge production comes mainly from the processing of zinc ore and the extraction of coal fly ash. Natural Ge consists of five isotopes: 70Ge (20.52%), 72Ge (27.45%), 73Ge (7.76%), 74Ge (36.52%), and 76Ge (7.75%). Ge is a semiconductor material, which has a variety of applications in industry, including electronics, fiber optic systems, infrared optics, polymerization catalysts, and solar applications. Commercially available Ge ingots have impurity levels ranging from 99.99% to 99.999%. Even the highest commercially available impurity level is unacceptable for growing Ge crystals for making Ge detectors. Therefore, commercial ingots must be purified to a level of 99.999999999.9% to grow a Ge crystal, part of which has a chance of reaching the level of impurity needed to produce a detector grade crystal with an impurity level of 99.999, 999.999.99%.

Ge purification

The Ge is purified using a process called zone refining, which creates a melting zone to pass from one end of the Ge ingot to the other. This process was first developed at Bell Laboratories in 1954. During the zone refining process, impurities are separated at the liquid-solid boundary of the melting zone. The impurities remain in the melt and move towards the end. After many passages, the impurities concentrate in the end of the Ge ingot. Once the end part is cut off, a large part of the Ge ingot sees its purity increased. Depending on quality control (the optimization of multiple influential parameters) and the number of passes, zone refining can purify Ge ingots from 99.999% to 99,999,999,999.9% – a reduction of seven orders of magnitude. The purified Ge ingots can then be used for growing Ge crystals. After 12 years of surface laboratory research and development (R&D), the University of South Dakota (USD) has established a standard procedure that ensures a high yield (~80%) of purified Ge ingots meeting the requirements ( seven orders of impurity magnitude reduction) of crystal growth.

Shown is the purification of Ge ingots through a zone refining process

Growth of Ge crystals

Large Ge crystals are grown with the Czochralski technique, which was developed in the 1930s. Crystal growth is an art. Crystal growth involves phenomena of heat, momentum and mass transport, chemical reactions (eg, contamination of crystals and melt), and electromagnetic processes (eg, heating by induction and by resistance, magnetic agitation, magnetic breaks, etc.). Therefore, the process of dynamic crystal growth involves a phase transformation from liquid to solid. The control of the interface between the liquid and the solid is on the scale of the nanometer and the growth system is of the order of the size of a meter. This requires many parameters (ten or more) to optimize with many constraints. Therefore, the dynamic character of the crystal growth process is very difficult to control. The growth rate and quality of high-purity Ge crystals largely depend on the control of the thermal field (heat transfer and temperature profile). However, these control parameters can only be adjusted externally. Specifically, they can only be regulated by the geometry of the growth system, gas flow and pressure, draw rate, frequency, and power of the RF heater. Measurements inside the growth chamber (temperature above 1000ohC), and a quantitative determination of the control parameters are technically difficult. With 12 years of surface lab R&D, USD invented a growth method that facilitated the consistent production of detector-grade (99,999,999,999.99%) crystals. This paves the way for the underground growth of Ge crystals.

Crystal characterization

Characterization of the grown crystals provides feedback for the growth process and helps determine if the crystals are of sufficient quality to be used in detector fabrication. With 12 years of R&D experience, USD has created a rich characterization program to determine which crystals meet the qualifications for detector fabrication. The quality of a single crystal Ge crystal is determined by X-ray diffraction and dislocation density measurements using an optical microscope. The impurity level is determined by Hall effect measurements; the purity of the grown crystal is confirmed by identifying shallow impurities using Photothermal Ionization Spectroscopy (PTIS) and natural impurities with Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Additional knowledge and techniques for characterizing grown crystals can be obtained using various established characterization techniques.

An advanced Hall van der Pauw measurement system, capable of testing samples at varying temperatures from 77K to 300K, can be realized. We can also use a deep level transit spectroscopy system which will allow the identification of deep level impurities in the grown crystals. This set of characterization tools allows us to have a comprehensive understanding of developed crystals to develop new detectors, whose performance is highly dependent on crystal properties, including shallow/deep level impurities, dislocation density and the defects.

Manufacture of the detector

Using the developed crystals, USD and his collaborators can fabricate Ge detectors in different geometries, including planar detectors, point-contact detectors, and ring-contact detectors with different contact technologies. Commercially available Ge detectors are fabricated with lithium diffusion as the n+ contact and boron implementation as the p+ contact. The basic principle is to create a load barrier height using traditional pn junction technology.

crystal growth
A mechanically processed Ge detector with ring contact geometry using a USD-grown crystal at Texas A&M University

More recently, scientists at Lawrence Berkeley National Laboratory invented bipolar blocking technology using amorphous Ge and amorphous Si as sensing contacts. This technology creates thin contacts (~600 nm) on Ge compared to the thick contact diffused by Li (~1 mm). One of the advantages of fine contacts is to produce segmented Ge detectors. USD has the ability to manufacture thin contact detectors using sputter technology. After five years of R&D, USD has established detector manufacturing capability and demonstrated thin contact technology with crystals developed by USD.

SHU Surface Laboratories

The in-house production line of Ge detectors from Ge zone refining, crystal growth, crystal characterization and detector fabrication has been established at USD since 2010. State subsidy of South Dakota established the Governor’s Research Center for Ultra-Low Background Experiments in the Dakotas (CUBES). Thanks to these funds, USD was able to start exploring the growth of Ge crystals in surface laboratories. Shortly thereafter, a Department of Energy Established Program Grant to Stimulate Competitive Research (EPSCoR) funded the Crystal Growth and Detector Development Program in South Dakota through its Implementation Award. This grant enabled USD to move quickly towards full R&D capability to build the Ge detector production line.

The South Dakota Board of Regents and USD invested significant funds to renovate approximately 6,600 square feet of laboratory space and facilitated the electrical, water, and gas lines needed to grow Ge crystals. and develop Ge detectors. More recently, the National Science Foundation funded a USD-led international team through Partnerships for International Research and Education (PIRE) to continue Ge crystal growth and detector development. All of the above has helped make the SHU surface labs available to transfer technology underground for Ge purification, crystal growth, crystal characterization and detector development.

Attention, this article will also appear in the twelfth edition of our quarterly publication.

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