Germanium single crystals have extensive application in nanoelectronics, detector engineering, and IR optics. In photovoltaics germanium is used as a substrate material for solar cells GaInP/GaInAs/Ge type with the conversion efficiency up to ∼ 39 % [1–5]. It requires low impurity dislocation-free Ge-crystals since dislocations and uncontrolled impurities can cause a mismatch between the lattice parameters of germanium and A I I I B V compounds, impeding the growth of high-quality epitaxial layers on germanium [1, 3–4].
High carrier mobility in dislocation-free Ge promotes its application in the radiation- hardened power MOSFET transistors and other fast digital electronics of the space class . Dislocations and foreign impurities limit the use of germanium in infrared optics, since they change the optical properties of germanium . Recent studies have proved the oxygen to be one of the main impurities affecting structural perfection, electrical and optical properties of germanium single crystals, and operational characteristics of Ge-based electronics [7–17].
Oxygen atoms dissolved in germanium are located at interstitial positions O i . This defect in the simplest model may be regarded as a nonlinear symmetric quasi-molecule Ge–O–Ge, with vibration modes ν 1, ν 2, and ν 3. It is the general practice to determine oxygen concentration in the crystals by the absorption peak at 856 cm−1 in the infrared spectra, which is identified with asymmetrical vibration mode ν 3 [7–9]. In the recent research [10–12] the oxygen peak at 856 cm−1 has been reported to have a “shoulder” at , that equals 843 cm−1, which is also identified with O i vibrations.
Oxygen is known to exist in atomic and bond states, e.g. in the form of precipitates GeO x , formed either during crystal growth or due to decomposition of supersaturated solid solution of oxygen in the process of post-growth annealing and cooling. The oxygen content in precipitates can achieve 20 % of its total concentration. It is established that the annealing of Germanium at the temperatures from 350 to 450 ∘C leads to the formation of the precipitates with electric activity, named thermal donors (TD). The majority of modern TD models are based on the idea that these centers are complexes with an electrically active core and different bounded numbers of oxygen atoms [4, 13–16].
It should be noted that oxygen concentration in the crystals investigated in the studies [7–16] is typically ∼1017 cm3 or higher. Thus, germanium was oxygen enriched.
In this regard, the study aims to investigate germanium single crystals with lower oxygen concentration in the order of 1016 cm−3, the annealing impact in the temperature range from 350 to 450 ∘C on the behavior of the oxygen, the form of its existence in germanium and the optical properties of single crystals depending on the oxygen partial pressure ranging from 10−3 to 103 Pa.
The studies were made on Sb-doped Ge single crystals with a donor concentration between 1.1·1015and 7.0·1014 cm−3, corresponding to a specific electric resistivity in the range of 2 to 4 Ohm·cm. The material with dislocation content about ∼104 cm−2 has absorption coefficient α of 0.15-0.20 cm−1 at the wavelength of 10.6 μm [4, 5].
Ge crystals were grown by the Czochralski method using a graphite crucible in argon atmosphere. Polished plane-parallel plates with the thickness of metricconverterProductID1 cm1 cm were prepared to study optical properties with Fourier transformed infrared spectrometry. The infrared measurements were performed in the 600-4000 cm−1 spectral range with SPECTRUM BXII spectrometer. The optical density measurement error was no more than ± 0,001.
The oxygen concentration [O i ] was calculated from the measured amplitude of the absorption band at 843 cm−1 using the formula:
(1) where D – is the optical density relative to the baseline; d – is the sample thickness; 1.05·1017 cm2 – is the calibration factor .
The influence of isothermal annealing in gas with P O2 ranging from 103 to 103 Pa on the optical properties of single crystals was determined by the optical density value and absorption coefficient α at a wavelength of 10.6 μm. For IR measurement with the temperature rising up to 60 ∘C the heat device ensuring a stable sample thermostating with accuracy ± 0.1 ∘C was used.
Fig. 1 shows IR absorption spectra of Sb-doped Ge single crystals with specific electrical resistance of 3 Ohm·cm at room temperature and heated up to higher temperatures in the range of wave numbers from 600 to 1500 cm−1.
Only one absorption peak 843 cm−1 was observed at the range of wave numbers from 800 cm−1 to 900 cm−1 (Fig. 2). We identify the peak as the “oxygen” band [10, 17]. The oxygen concentration in the germanium crystals under study was determined by the value of optical density in this band maximum as ∼ 1.10·1016 cm3.
The oxygen band position in Ge IR spectrum is disputable. We can suggest considering the findings of the experiments in the studies [10-12, 17] that the absorption band position corresponding to the vibrational mode of O i atoms can be 843 or 856 cm−1, depending on their content in the crystal.
This hypothesis was confirmed by the results of the influence of diffusion annealing on optical properties of Ge crystals. The annealing was conducted in gas, containing residual oxygen with the partial pressure range from 1 to 103 Pa and in the temperature interval from 350 to 450 ∘C with an intermediate registration of infrared spectra data. Fig. 3 shows Ge IR spectra after annealing at P O2 ≈ 103Pa at temperature 400 ∘C. It was found that absorption intensity at wavelength 843 cm−1 increased after the four-hour period annealing with the oxygen concentration increase from ∼1.10·1016 to ∼1.30·1016 cm−3. The six-hour or eight-hour period annealing leads to even greater increase of oxygen concentration in the crystal. It results in a new band at wavelength 856 cm−1 that corresponds to O i vibrations.
Therefore, annealing at 400 ∘C and partial pressure range from 1 to 103 Pa is responsible for the increase of oxygen concentration in Ge and a new absorption band appearancewith the increasing intensity at the wavelength856 cm−1in addition to band 843 cm−1.
The experiment has proved that annealing at a lower oxygen partial pressure of P O2 < 1 Pa leads to the reduction of band intensity 843 cm−1 that corresponds to the content decrease of O i . The 90- hour period annealing at P O2 ≈ 10−3 Pa and temperature 400 ∘C in Krypton (6N) leads to the oxygen band 843 cm−1intensity reduction on 5 %.
The phenomenon observed in a similar way in papers [11, 16] can be explained by the formation of thermal donors (TD) based on dissolved oxygen in the germanium crystal lattice during annealing. Thus, the part of dispersed oxygen is bound in TD structure due to annealing. It was found that the germanium annealing at low oxygen partial pressure ∼ 10−3 Pa resulted in the change of optical characteristics of crystals, such as optical density and absorption coefficient.
The data in Fig. 1 show that the optical density of the sample with electrical resistance of 3 Ωm·cm at the room temperature and wavelength 10.6 μm is 0.337. When the temperature increases up to 60∘C, the optical density increases to 0.365. D change matches the absorption coefficient increase from 0.017 to 0.065 cm−1. This temperature destabilization of germanium optical properties impedes its application in IR optics at overheating to 45 ∘C.
Table 1 shows the optical properties of the samples annealed at temperature 400 ∘C and P O2 ≈ 10−3 Pa during 90 hour-period. It was found that optical density of these samples at 60 ∘C decreases from 0.365 to 0.360 after annealing at low oxygen partial pressure and the absorption coefficient decreases from 0.066 to 0.056 cm−1. On average the absorption coefficient of Ge single crystals at 60 ∘C decreases by 13 % for the samples with electrical resistance in the range from 2 to 4 Ωm·cm as the result of annealing. The annealing influence on the optical properties of crystals at the room temperature was insignificant.
The experimental data show that the annealing of Ge single crystals at 400 ∘C and P O2 ≈ 10−3 Pa provides the temperature stability of their optical properties. It is assumed that the temperature stability improvement of the optical properties is due to oxygen concentration decrease and TD formation.
The annealing at the oxygen partial pressure ranging from 10−3 to 103 Pa in the temperature range from 350 to 450 ∘C leads to the change of concentration and existence form of oxygen dissolved in Ge crystals with oxygen content of ∼1016 cm−3. The oxygen concentration is increased after annealing at partial oxygen pressure from 1 to 103 Pa. The oxygen band maximum shifts from 843 to 856 cm−1 when its concentration increases.
Annealing of Ge crystals at P O2 ≈ 10−3 Pa leads to the oxygen band 843 cm− 1 intensity reduction due to TD formation. The decrease of unbound oxygen and TD formation are accompanied by the improvement of the temperature stability of the crystal optical properties.