Experimental studies on the temperature dependence of Germanium semiconductor detectors and on their pixilation for high spatial resolutions

半导体探测器中电子-空穴对热产生的噪声电流基本受Boltzmann因子exp（-E_< The >/kT）的支配，其中E_< The >为特征能，k为Boltmann常数，T为绝对温度。硅半导体探测器可以在室温下工作，因为在T=25℃时，硅中的电子空穴对密度n_<e-h>的数量级为10^<10> cm^<-3>，这是由于E_<th>(1.1 eV)的高值。相反，在T=25℃时，由于E_<th>(0.7 eV)值较低，锗中的n<e-h>高达10^<13> cm^<-3>，使得锗半导体探测器必须在低温条件下工作。然而，在中等低温条件下（T=-65℃），锗中的n_<e-h>已经与室温下硅中的n_<e-h>一样低。此外，在本研究中假设的像素尺寸（100 μm × 100 μm）的电极上，噪声电流应降低到100 nA / pe以下，因为它与每个像素电极覆盖的半导体体积成正比。另一方面，电离粒子产生的电子-空穴对的密度数与探测体积无关，随着电离粒子能量的增加而增加，并局限在几百μm以内。这一论点表明，当像素化在几百μm时，锗半导体可以在室温附近工作。本研究证实，在体积为1mm × 1mm的微小锗探测器中，噪声电流在室温下已降至10ma，在T=-73℃时进一步降低了两个数量级。这意味着，当像素化锗晶体达到0.2 mm × 0.2 mm × 0.3 mm的标准尺寸时，噪声电流将降低到30 nA以下，并支持像素化锗探测器在近室温下工作的可能性。然而，当施加电压高于10 V时，观察到暗电流，这可能是表面泄漏电流。因此，最新版本的像素化锗探测器的晶体体积为5毫米× 5毫米，像素尺寸为0.75毫米× 0.75毫米，像素数为2 × 2，还有一个用于去除暗电流的保护环电极。少

The noises current originated by thermal generation of electron-hole pairs in semiconductor detectors is basically governed by the Boltzmann factor of exp(-E_<th>/kT), where E_<th> denotes the characteristic energy, k Boltmann constant, and T the absolute temperature. Silicon semiconductor detectors can be operated at room temperature, because the number of density of electron-hole pairs, n_<e-h>, in silicon is on he order of 10^<10> cm^<-3> at T=25℃ owing to the high value of E_<th>(1.1 eV). On the contrary, n<e-h> in germanium is as high as 10^<13> cm^<-3> at T=25℃ due to the low value of E_<th>(0.7 eV), making the cryogenic condition compulsory to the operation of germanium semiconductor detectors. However, n_<e-h> in germanium at moderate cryogenic condition (T=-65℃) is already as low as that in silicon at room temperature. With an electrode as small as the pixel size assumed in the present work (100 μm × 100 μm), furthermore, the noise current should be reduced less than 100 nA pe … More r pixel, since it is proportional to the volume of the semiconductor each pixel electrode covers. The number of density of electron-hole pairs generated by ionizing particles is, on the other hand, independent from the detection volume, increases with the energy of ionization particles, and localized within a few hundreds μm. This argument suggests that, when pixilated in a few hundreds μm, the germanium semiconductor can be operated near room temperature. The present study has confirmed that the noise current is already reduced to 10 mA at room temperature in a tiny germanium detector with a volume of 1 mm × 1mm, and is further reduced by two orders of magnitude at T=-73℃. This implies that the noise current will be reduced below 30 nA with a germanium crystal pixilated into a proposed standard size of 0.2 mm × 0.2 mm × 0.3 mm, and supports the operation possibility of a pixilated germanium detector at near room temperature. However, there was a dark current observed with an applied voltage higher than 10 V, which is presumably a surface leakage current. The latest version of pixilated germanium detectors fabricated has, therefore, a crystal volume of 5 mm × 5 mm, a pixel size of 0.75 mm × 0.75 mm, and a pixel number 2 × 2, together with a guard-ring electrode that is to remove the dark current. Less