Research On Chemical Passivation Of Single-crystalline Germanium

Germanium oxides, especially GeO2, due to their poor electronic properties and instability in air or other oxidants, have restricted the extensive development in germanium semiconductor applications. However, substochiometric GeO_x films have attracted much attention of researchers recently. The reported work was mainly focused on the application of GeO_x as a precursor for the formation of germanium nanocrystal. However, there are very few reports on the using of GeO_x as a passivant to the germanium crystal.In this thesis, wet chemical methods were applied to achieve thin GeO_x films on the germanium surface. According to the minority carrier lifetime and X-ray photoelectron spectrum (XPS) tests, systematical analysis on the effect of different factors on the passivation performance was carried out. Also, two different wet chemical treatments were compared. Theoretical simulation was developed on the basis of random-bonding model and the Sanderson electronegativity method to explain the mechanism of the passivation effects achieved by the wet chemical method. The main results in this work are summarized as follows:1. Oxidants-only treatment. n-type and p-type germanium wafer were oxidized by oxidants: nitric acid solution and peroxide solution, respectively. The maximum minority carriers'lifetime values for n-type and p-type germanium wafer were 75μs and 50.27μs, respectively (n: 49.57μs, p: 32.8μs for untreated samples). XPS results were used for further mechanism analysis. The bonding between Ge atom and O atom has reduced the density of dangling bond, which in turn decreases the density of combination energy level and realizes the passivation effects. XPS results also revealed that the passivation effects were not resulted from GeO2 but from the other subvalent state of Ge atom in GeO_x (1<x<2) films.2. Oxidant and hydrofluoric mixed solution treatment. Samples of two conduction types (n, p) were treated by mixed solution of peroxide solution and hydrofluoric solution. The maximum minority carriers'lifetime values for n-type and p-type germanium wafer were 158μs and 200μs, respectively (n: 49.57μs, p: 32.8μs for untreated samples). According to the XPS results, affected by the oxidizability of peroxide and selective solubility of hydrofluoric acid solution, the minority carriers'lifetime first increased then decreased as x value in GeO_x films increases. The optimum passivation effect can be achieved when x is in the range of 1~1.5, i.e., +2~+3 for the chemical valence of Ge atom.3. XPS results indicate that: hydrofluoric acid solution just played a solution role in the oxidizing-then-hydrofluoric treatment. In mixed solution treatments, by controlling the volume percentage of hydrofluoric acid, the x value in the resultant GeO_x film can be controlled, which may generate different passivation effect.4. Theoretical simulation. As x value in GeO_x increased, relative contents and binding energies of five different sub-units were calculated. Combined with the experimental results, relative contents of each sub-unit were also obtained in samples with better passivation effects.