Ion-beam mixing in crystalline and amorphous germanium isotope multilayers

Abstract

Gallium (Ga) implantation induced self-atom mixing in crystalline and amorphous germanium (Ge) is investigated utilizing isotopically controlled Ge multilayer structures grown by molecular beam epitaxy. The distribution of the Ga ions and the ion-beam induced depth-dependent mixing of the isotope structure was determined by means of secondary ion mass spectrometry. Whereas the distribution of Ga in the crystalline and amorphous Ge is very similar and accurately reproduced by computer simulations based on binary collision approximation (BCA), the ion-beam induced self-atom mixing is found to depend strongly on the state of the Ge structure. The experiments reveal stronger self-atom mixing in crystalline than in amorphous Ge. Atomistic simulations based on BCA reproduce the experimental results only when unphysically low Ge displacement energies are assumed. Analysis of the self-atom mixing induced by silicon implantation confirms the low displacement energy deduced within the BCA approach. This demonstrates that thermal spike mixing contributes significantly to the overall mixing of the Ge isotope structures. The disparity observed in the ion-beam mixing efficiency of crystalline and amorphous Ge indicates different dominant mixing mechanisms. We propose that self-atom mixing in crystalline Ge is mainly controlled by radiation enhanced diffusion during the early stage of mixing before the crystalline structure turns amorphous, whereas in an already amorphous state self-atom mixing is mediated by cooperative diffusion events.

Temperature dependence of ion-beam mixing in crystalline and amorphous germanium isotope multilayer structures

“…This mixing behavior supports the predictions of MD calculations and contradicts our former results obtained by using MBE grown amorphous isotope layers. 10 The opposite result of our former work published in Ref. 10 is likely affected by contaminations of oxygen and carbon later detected with secondary ion mass spectrometry (SIMS).…”

“…10 The opposite result of our former work published in Ref. 10 is likely affected by contaminations of oxygen and carbon later detected with secondary ion mass spectrometry (SIMS). Our measurements reveal that oxygen and carbon are easily incorporated in MBE grown amorphous Ge by handling in air.…”

“…In this case, both structures were grown by means of molecular beam epitaxy (MBE). 10 A stronger mixing in the crystalline MBE multilayer structure compared to its amorphous counterpart was observed for 300 K. This behavior is at variance to molecular dynamics (MD) simulations of, e.g., Nordlund et al 18 In order to test our former results, we prepared an amorphous isotope multilayer from the MBE grown crystalline isotope structure by Ge implantation. We used this preamorphized Ge structure for comparison of ion-beam mixing in crystalline and amorphous Ge induced by Ga implantation at different temperatures.…”

“…Present studies mainly concern ion-beam induced self-atom mixing in Si and Ge at room temperature. [8][9][10] In our previous work, we described ion-beam induced self-atom mixing in crystalline and amorphous Ge isotope multilayer structures. In this case, both structures were grown by means of molecular beam epitaxy (MBE).…”

“…Such studies concern, e.g., the mechanisms of self-and dopant diffusion, [1][2][3] the electronic properties and nature of deep defect centers, 4 the impact of isotopic composition on heat transport, [5][6][7] and the ion-beam induced self-atom mixing. [8][9][10] Although atomic mixing in solids caused by ion irradiation is a known and widely discussed phenomenon, [11][12][13][14][15][16][17] experimental results on self-atom mixing in semiconductors such as Si and Ge are rather limited. Present studies mainly concern ion-beam induced self-atom mixing in Si and Ge at room temperature.…”