Fabrication of n+ contact on p-type high purity Ge by cathodic electrodeposition of Li and impedance analysis of n+/p diode at low temperatures

The fabrication of diodes by forming n-type electrical contact on germanium (Ge) and its AC impedance analysis is important for radiation detection in the form of pulses. In this work, lithium (Li) metal has been electro-deposited on p-type Ge single crystal from molten lithium nitrate at 260 °C. The depth of Li diffusion in Ge was successfully varied by changing the electroplating time as determined by sheet resistance (SR) measurement after successive lapping of the Ge surface. Li is found to diffuse up to 500 µm inside Ge by heat treatment of as-deposited Li/Ge at 350 °C for 1 h. A stable n-type electrical contact on Ge with SR ~ 1 Ω/□ and impurity concentration ~ 3.7 × 1015/cm3 is developed by Li incorporation in p-type Ge crystal showing net carrier concentration ~ 3.4 × 1010/cm3 and SR ~ 100 KΩ/□. Carrier concentration determined from the 1/C2 vs. V plot shows similar temperature dependence as found by Hall measurement. The fabricated n+/p junction exhibits excellent diode characteristics with a gradual increase in cutoff voltage at low temperatures. Under forward bias, junction capacitance mainly comprises diffusion capacitance (~ 10 µF), showing strong frequency dependence, and the impedance is partly resistive, resulting in a semi-circular Cole–Cole plot. Imaginary impedance spectra reveal that the relaxation time for the diffusion of majority carriers decreases at higher temperatures and increased forward voltages. The diode is purely capacitive under reverse bias, showing a line parallel to the y-axis in the Cole–Cole plot with frequency-independent (100 Hz–100 MHz) depletion capacitance of ~ 10 pF.