Chapter 8 Hyper-Pure Germanium Detector

1 Med Phys 4RA3, 4RB3/6R03 Radioisotopes and Radiation Methodology 8-1 Chapter 8 Hyper-Pure Germanium Detector Introduction Silicon semiconductor detectors described in the previous Chapter have depletion depths less than 1 mm, which are sufficient for charged particle spectroscopy or soft X-ray detection. For photon spectroscopy in the energy region of hundred keV or several MeV, much thicker semiconductor detectors are required. Fig. shows the mean free path of a photon in silicon and Germanium . It is obvious that a depletion depth of at least several cm is required. Due to its higher atomic number, Ge has a much lager linear attenuation coefficient, which leads to a shorter mean free path. Thus, Ge is preferred for hard X-ray or gamma-ray detection to achieve higher detection efficiency.

2 10110210310410-310-210-1100101 GeSi Photon energy [keV]Mean free path [cm] Fig. Photon mean free paths in Si and Ge. For a semiconductor Detector , the depletion depth is given by 2/10)2(eNVd where, V0 is the reverse bias voltage and N represents the net impurity concentration in the initial semiconductor material. If silicon or Germanium of normal semiconductor purity is employed, the maximum achievable depletion depth is a few mm even at bias voltages close to the breakdown level. Thus, the impurity concentration should be much reduced down to 1010 atoms/cm3 in order to realize intended depletion depths of cm order. At this impurity concentration, a reverse bias voltage of 1 kV can produce a depletion depth of 1 cm. The required impurity concentration corresponds to levels less than 1 part in 1012, which is quite challenging.

3 One way to further improve the net impurity concentration is to compensate the residual impurities with an opposite type impurity material. In both Si and Ge, the material with highest available purity tends to be p-type, which requires addition of donor atoms for compensation. The alkali metals like Li, Na tend to form interstitial donors in Si and Ge. When one of these donor materials (practically Li) is introduced into Si or Ge, the donor atoms are easily ionized and are mobile enough to drift at elevated temperatures under the influence of a strong electric field. This process is so-called lithium ion drifting and has been applied in both Si and Ge. For convenience, Ge(Li) represents lithium drifted Ge and Si(Li) means lithium drifted Si. The Med Phys 4RA3, 4RB3/6R03 Radioisotopes and Radiation Methodology 8-2 lithium mobility is much greater in Germanium and remains high enough at room temperature, which leads to an undesirable redistribution of the lithium.

4 Therefore, the lithium profile of Ge(Li) detectors must always be preserved at LN2 temperature while Si(Li) detectors can be stored at room temperature for a short period due to low mobility of the Li ion in silicon. The other way to improve the net impurity concentration is to add additional refining processes so that the intended purity of the crystal can be met. Techniques have been developed to achieve high purity Ge crystals in this approach and HyperPure Ge (HPGe) detectors are commercially available. However, no equivalent technique is available for Si yet. Ge(Li) detectors were popular in the past and served as the common type of large-volume Germanium Detector for a long time. Although there is little difference in Detector performance between HPGe and Ge(Li), the maintenance of Ge(Li) is pretty inconvenient, which has made manufacturers stop producing them.

5 Configurations A. Hyper-Pure Germanium (HPGe) Detector fabrication [4] HPGe crystal were first developed in the mid 1970s. The starting material is bulk Germanium intended for the semiconductor industry. Although already of very high purity, the material is further purified with the zone refining technique. The Germanium is melted in a crucible using radio-frequency (RF) heating coils. The underlying principle is that impurities concentrate in the liquid phase leaving the solid purer than the original melt as a liquid freezes and solid appears. Fig. Three coil zone refine. Fig. Growing Ge crystal. As shown in Fig. , each zone refiner coil melts a small section of the Germanium in the crucible. As the RF coils are slowly moved along the length of the crucible, the molten zone moves with them.

6 Thus, the Germanium melts as the coil approaches and freezes as the coil moves away. The impurities tend to remain in the molten section, which leads to a higher concentration of impurities in the liquid than the solid. In this way, the impurities are swept to one end. This sweeping operation is repeated many times, until the impurities are concentrated at one end of the ingot. This end is then removed, leaving the remaining portion much purer than Med Phys 4RA3, 4RB3/6R03 Radioisotopes and Radiation Methodology 8-3 the original starting material. The improvement or reduction in impurity concentration actually realized is about a factor of 100 or more at the completion of this process. Large single crystals of Germanium are grown using the Czochralski technique.

7 A precisely cut seed crystal is dipped into the molten Germanium and then withdrawn slowly, while maintaining the temperature of the melt just above the freezing point as shown in Fig. The rate of crystal withdrawal and temperature of the melt are adjusted to control the growth of the crystal. If the remaining extremely low-level impurities are acceptors, the property of the crystal is mildly p-type while high purity n-type is the result in the other case. B. Planar configuration An example of a planar HPGe Detector using a p-type crystal is shown in Fig. In this configuration, the electric contacts are provided on the two flat surfaces of a Germanium crystal. mm thick Ben+ contact ( mm Li)Photonsehp-typep+ ( m B)p-n junction Fig. Planar HPGe Detector (p-type). A lithium evaporation and diffusion to form the n+ contact is performed over one of the surface.

8 This lithium-diffused layer is about several hundred m thick. The depletion region is formed by reverse biasing n+-p junction. The surface of the opposite side is modified to the p+ layer by ion implantation of boron acceptor atoms to increase the conductivity near the surface. Since both materials are p-type, no semiconductor junction exists at this side. Instead, the p+ layer provides the electric contact to collect the charge carriers created by the radiation. The thickness of the implanted boron layer is so thin (a few tenths of m) that it is a suitable entrance window for low energy photons. To make a reverse bias, a positive high voltage is applied to the n+ contact with respect to the p+ surface. The depletion region is formed at the region close to the n+ contact and then expanded deeply into the p-side as the bias voltage is raised.

9 Once the Detector is fully depleted, further increase of the bias (overvoltage) does not make any effect on the sensitive volume, however, it makes the electric field stronger, which subsequently shortens carrier collection times and reduces the risk of carrier losses (recombination and trapping). Med Phys 4RA3, 4RB3/6R03 Radioisotopes and Radiation Methodology 8-4 Planar HPGe detectors can also be fabricated starting with high-purity n-type crystals. Similarly, n+ and p+ contacts are provided on both sides. Reverse biasing also requires the application of a positive voltage to the n+ side. In this case, the depletion region is expanded from the p+ contact. When fully depleted and operated with a sufficient overvoltage, the electric field inside the crystal is almost uniform and charge carriers drift under a constant electric field.

10 C. Coaxial configuration From the photon mean free path in Germanium , a Detector thickness of the order of 5 cm is required for efficient detection of MeV photons. The maximum depletion depth for the planar detectors is limited to less than 1 or 2 cm. To produce a Detector with a thicker depletion depth, a different electrode configuration must be employed. HPGe detectors dedicated for MeV photons are constructed in coaxial geometry as shown Fig. In this configuration, one electrode is fabricated at the outer surface of a cylindrical crystal and the other electrode is located at the inner surface of the central hole. In this way, much larger active volumes can be achieved. True coaxialClosed-ended coaxial Fig. Large volume coaxial HPGe detectors . A closed-ended configuration is one in which only part of the central core is removed and the outer electrode is extended over one flat side surface.