**3.2 Geometry and process**

The structures that have been used from the early days of Ge detectors are p+n−n+ and p+p−n+, usually with a Li-diffused n+ contact and a boron-implanted p+ contact, and a thin metallization of sputtered aluminum or electrolytically plated gold in old detectors. Following an appropriate surface treatment (etching with CP4 mixture after cleaning with solvents such as acetone) and rinsing with a slightly oxidizing mixture, the detectors are placed in secondary vacuum, and after surface desorbing, the leakage current can be stabilized at low values. The leakage dark current at 77 K with a reverse bias of 1 kV or more for large coaxial detectors can be reduced to the order of a few pA or less. Usually a thermal treatment above room temperature will allow surface desorbing, and consequently impurities on the surface will be eliminated. The question of passivation is still open, as germanium does not have the self-passivation properties of silicon, for which oxygen creates a stable oxide layer of a few nm at room temperature. Germanium oxides are not stable [26–29] and react with water. More recent passivation methods are based on amorphous layer deposition such as a-Ge-H (amorphous hydrogenated germanium). These constitute a coating, rather than standard passivating layers. In spite of this, their effect is to stabilize leakage currents at 77 K at reasonable values. However, these values are higher than those for bare Ge material in vacuum.

As the presence of defects mainly alters the transport of holes, efforts were made to reduce the mean-drift length of holes by an adequate electrode configuration [30]. For coaxial detectors, the hole-collecting electrode can be placed at the outer surface of the detectors. This is usually a p+ outside implant, which is shallower than the n+ electrode, which is placed on the axis of the detector. Holes have a shorter drift distance than the electrons, the p+ electrode being negatively biased. This configuration is less sensitive to cumulative nonionizing radiation effects. N-type material is used contrary to p-type material with opposite electrode configuration in the conventional detectors. The same is true for planar detectors, for which the collecting length is reduced to values well below 1 cm. The cumulative radiation effects are mainly deep defects introduced by irradiation by nonionizing particles, such as hadrons (particularly neutrons and protons), but also energetic electrons. These particles, particularly at energies in the MeV range,

**73**

electrostatic effect.

*High Purity Germanium: From Gamma-Ray Detection to Dark Matter Subterranean Detectors*

induce displacement cascades in the detecting crystalline material and therefore produce defects of various nature, point-like or clusters. The temperature of operation and irradiation has a great influence on the outcome [25]. The result is a degradation of the energy resolution, which can be observed as a broadening of the photo-peaks, with a tail at low energy giving them an asymmetric aspect. As the detector can be compared to an ionization chamber with two electrodes polarized at different potentials, the current integrated by the readout electronics is a displacement current. The total charge collection is achieved when all holes and electrons are collected by the n+ (positively biased) and p+ (negatively biased) electrodes (cathode and anode). If carriers become trapped during the transport process, a loss of charge occurs leading to a peak tail at low energy. The collected charge is lower than the photo generated charge. Statistically, there is a certain charge deficit in the signal corresponding to one event, so the high-energy side of the peaks is not much affected [30], contrary to the low energy side. This leads to a loss of energy resolution. The charge loss is often followed by charge reemission with a large time constant, which has no real influence of the spectrum. An analytical model of radiation-induced defects has been proposed in Ref. [25] and

The radiation-induced effects have been widely studied in HPGe detectors [30, 25, 31], and, in the beginning, without a quantitative relation to crystalline defects introduced by irradiation in HPGe. Since these effects result in resolution degradation, their characterization is of utmost importance. We can cite a few extensive works on this subject, mostly using electrical measurements [12, 13]. A powerful characterization technique called photoelectric spectroscopy [32] or alternatively photo-thermal ionization spectroscopy (PTIS) has been successfully applied to HPGe, but this technique is limited to shallow hydrogenoid levels with low concentration and not operation-detrimental deep levels. It applies a two-step process, phonon + photon (far infrared) and requires low temperature operation (LHe) below the ionization temperature of impurities and dopants. No overlapping of the bound electron wavefunction is required, so it can only be used when the impurities have a low concentration. For deep-defects, deep level transient spectroscopy (DLTS) became the technique of choice. In particular, it helped to establish that deep traps that are created by a temperature increase above 77 K are more detrimental to detector operation than the primary defects that are created by irradiation at low temperature [3]. Neutron-induced defects are thought to be vacancy and interstitial related at 77 K. After annealing above this temperature, divacancies and impurity-vacancy defects are the most numerous centers observed that are electrically active. They give rise to numerous deep hole-traps with capture

where less electrically active defects are created. The stable defects at 100 K and less are disordered regions containing a large concentration of vacancies and interstitials. These act as hole-traps but are less effective in degrading resolution than secondary defects observed at room temperature. Numerous studies have been devoted to the degradation of the resolution, and most of them identify the causes as being related to the defects with capture cross sections of the order of 10<sup>−</sup>11 cm−<sup>2</sup> [2]. We know from experiments and simulations that these are due to zones with a high local defect density, which enhances capture probability [33] through an

Street [34] has found that the presence of disorders in amorphous silicon enhances the cross section for the capture of carriers by the defects. Later, an

. They only anneal out above 420–470 K,

*DOI: http://dx.doi.org/10.5772/intechopen.82864*

developed in later papers.

cross sections of the order of 10<sup>−</sup>13 cm−<sup>2</sup>

**3.3 Defects**

*High Purity Germanium: From Gamma-Ray Detection to Dark Matter Subterranean Detectors DOI: http://dx.doi.org/10.5772/intechopen.82864*

induce displacement cascades in the detecting crystalline material and therefore produce defects of various nature, point-like or clusters. The temperature of operation and irradiation has a great influence on the outcome [25]. The result is a degradation of the energy resolution, which can be observed as a broadening of the photo-peaks, with a tail at low energy giving them an asymmetric aspect. As the detector can be compared to an ionization chamber with two electrodes polarized at different potentials, the current integrated by the readout electronics is a displacement current. The total charge collection is achieved when all holes and electrons are collected by the n+ (positively biased) and p+ (negatively biased) electrodes (cathode and anode). If carriers become trapped during the transport process, a loss of charge occurs leading to a peak tail at low energy. The collected charge is lower than the photo generated charge. Statistically, there is a certain charge deficit in the signal corresponding to one event, so the high-energy side of the peaks is not much affected [30], contrary to the low energy side. This leads to a loss of energy resolution. The charge loss is often followed by charge reemission with a large time constant, which has no real influence of the spectrum. An analytical model of radiation-induced defects has been proposed in Ref. [25] and developed in later papers.

#### **3.3 Defects**

*Use of Gamma Radiation Techniques in Peaceful Applications*

50–60 K for a large emission rate window (56 s<sup>−</sup><sup>1</sup>

With σn higher than 10<sup>−</sup>13 cm−<sup>2</sup>

cm<sup>−</sup><sup>2</sup>

capture cross sections above 10<sup>−</sup>13 cm−<sup>2</sup>

with concentrations reaching 109

**3.2 Geometry and process**

be above 102

1 ms in duration (with a 1010 cm−<sup>3</sup>

with carrier capture cross section of the order of 10<sup>−</sup>13 cm−<sup>2</sup>

thumb [2] for the dislocation density that should not exceed 104

 cm<sup>−</sup><sup>3</sup> .

emission rate is large enough to have no marked effect on carrier trapping as the electrons are released with a time constant that is low compared with the drift time.

velocity at 50 K, in our DLTS measurements), the emission rate at 77 K exceeds the capture rate for an activation energy of 100 meV. The other fact is that the DLTS signal fades away above 65 K, so the contribution of the dislocation band should be low as long as the concentration remains at a reasonable level. This gives a rule of

1980s–1990s. If the dislocation density is too low, the deep impurities such as those related with copper (substitutional or bound with hydrogen) will be higher, as they cannot precipitate onto the dislocation lines and have a higher density than isolated impurities. They give rise to deep hole traps that affect greatly the hole transport even at 77 K. These traps have an activation energy higher than 0.160 eV and

The structures that have been used from the early days of Ge detectors are p+n−n+ and p+p−n+, usually with a Li-diffused n+ contact and a boron-implanted p+ contact, and a thin metallization of sputtered aluminum or electrolytically plated gold in old detectors. Following an appropriate surface treatment (etching with CP4 mixture after cleaning with solvents such as acetone) and rinsing with a slightly oxidizing mixture, the detectors are placed in secondary vacuum, and after surface desorbing, the leakage current can be stabilized at low values. The leakage dark current at 77 K with a reverse bias of 1 kV or more for large coaxial detectors can be reduced to the order of a few pA or less. Usually a thermal treatment above room temperature will allow surface desorbing, and consequently impurities on the surface will be eliminated. The question of passivation is still open, as germanium does not have the self-passivation properties of silicon, for which oxygen creates a stable oxide layer of a few nm at room temperature. Germanium oxides are not stable [26–29] and react with water. More recent passivation methods are based on amorphous layer deposition such as a-Ge-H (amorphous hydrogenated germanium). These constitute a coating, rather than standard passivating layers. In spite of this, their effect is to stabilize leakage currents at 77 K at reasonable values. However, these values are higher than those for bare Ge material in vacuum. As the presence of defects mainly alters the transport of holes, efforts were made to reduce the mean-drift length of holes by an adequate electrode configuration [30]. For coaxial detectors, the hole-collecting electrode can be placed at the outer surface of the detectors. This is usually a p+ outside implant, which is shallower than the n+ electrode, which is placed on the axis of the detector. Holes have a shorter drift distance than the electrons, the p+ electrode being negatively biased. This configuration is less sensitive to cumulative nonionizing radiation effects. N-type material is used contrary to p-type material with opposite electrode configuration in the conventional detectors. The same is true for planar detectors, for which the collecting length is reduced to values well below 1 cm. The cumulative radiation effects are mainly deep defects introduced by irradiation by nonionizing particles, such as hadrons (particularly neutrons and protons), but also energetic electrons. These particles, particularly at energies in the MeV range,

or more. The peak is at

of thermal

and should

) [3]. This means that at 77 K, the

cm s<sup>−</sup><sup>1</sup>

cm<sup>−</sup><sup>2</sup>

, (similar to coulombic/attractive centers)

, which is the case with a filling pulse shorter than

carrier concentration, and 106

for detector-grade material, at least for the material used in the

**72**

The radiation-induced effects have been widely studied in HPGe detectors [30, 25, 31], and, in the beginning, without a quantitative relation to crystalline defects introduced by irradiation in HPGe. Since these effects result in resolution degradation, their characterization is of utmost importance. We can cite a few extensive works on this subject, mostly using electrical measurements [12, 13]. A powerful characterization technique called photoelectric spectroscopy [32] or alternatively photo-thermal ionization spectroscopy (PTIS) has been successfully applied to HPGe, but this technique is limited to shallow hydrogenoid levels with low concentration and not operation-detrimental deep levels. It applies a two-step process, phonon + photon (far infrared) and requires low temperature operation (LHe) below the ionization temperature of impurities and dopants. No overlapping of the bound electron wavefunction is required, so it can only be used when the impurities have a low concentration. For deep-defects, deep level transient spectroscopy (DLTS) became the technique of choice. In particular, it helped to establish that deep traps that are created by a temperature increase above 77 K are more detrimental to detector operation than the primary defects that are created by irradiation at low temperature [3]. Neutron-induced defects are thought to be vacancy and interstitial related at 77 K. After annealing above this temperature, divacancies and impurity-vacancy defects are the most numerous centers observed that are electrically active. They give rise to numerous deep hole-traps with capture cross sections of the order of 10<sup>−</sup>13 cm−<sup>2</sup> . They only anneal out above 420–470 K, where less electrically active defects are created. The stable defects at 100 K and less are disordered regions containing a large concentration of vacancies and interstitials. These act as hole-traps but are less effective in degrading resolution than secondary defects observed at room temperature. Numerous studies have been devoted to the degradation of the resolution, and most of them identify the causes as being related to the defects with capture cross sections of the order of 10<sup>−</sup>11 cm−<sup>2</sup> [2]. We know from experiments and simulations that these are due to zones with a high local defect density, which enhances capture probability [33] through an electrostatic effect.

Street [34] has found that the presence of disorders in amorphous silicon enhances the cross section for the capture of carriers by the defects. Later, an analytical model has been developed that clearly explains this effect using simple assumptions [33]. This mitigates the direct role of disordered regions as being the sole origin of carrier capture at 77 K. The sizes of these disordered regions areof the order of the range of primary recoil atoms (**Figure 1**). Isolated defects should contribute greatly to the trapping process. The recoil of an atom is induced by the collision with the impinging particle. SRIM simulations show that its range is of the order of 10 nm at 10–30 keV, with around a few hundred vacancies being created on its trajectory. The recoil energy has been computed for neutrons in the MeV energy range, see **Figure 1**. In most cases, a thermal treatment above room temperature is used to remove radiation damage. In Ref. [31], recombination-enhanced annealing using minority-carrier injection was applied but with no significant results, at least at room temperature. At low temperature, when the defects are not stable, no improvement could be observed with this method. However, a strong dependence of the annealing stages on the material type (p or n) was observed in Refs. [3, 25] and other detector studies.
