In order to stabilize dark current and photocurrent under a higher negative bias, we used a slow-chopped He-Ne laser. At temperatures T < 180 K, both dark current and photocurrent are practically temperature independent. Both in the dark and under ∼ 100 mW / cm 2 He-Ne laser illumination, in the temperature interval from 300 to 180 K, we find exponential conductivity temperature dependencies, with activation energies of ∼ 200 and 60 meV, respectively. Dark current and photocurrent temperature dependencies under reverse bias of 0.5 V are summarized in Fig. At temperatures T < 150 K the reverse current stabilizes but decreases greatly to ∼ 10 − 11 A. At temperatures 150 K < T < 200 K, measurements are complicated by dark current and photocurrent instabilities.
At temperatures T > 200 K, we observe diodelike I - V characteristics with easily detectable photoconductivity. We confirmed that the majority of Ge NWs on a ( 100 ) Si substrate formed an angle of ∼ 55 ° to the Si substrate normal, while on a ( 111 ) Si substrate they grew nearly perpendicularly to the substrate surface with a small inclination of ∼ 4 °, most likely due to the substrate surface off-cut.ĭrastically different I - V behavior is found in Ge NW / ( n + ) Si samples. Finally, the samples were cooled in H 2 and then N 2 to < 200 ° C to minimize oxidation of the Ge NW surfaces. Their length depends linearly on the deposition time samples were prepared with Ge NW lengths of 360, 710, and 1400 nm. Typical diameter of the grown Ge NWs was 40 nm. Then, the temperature was reduced to 320 ° C, and GeH 4 was introduced into the chamber. After inserting the substrates into the lamp-heated CVD reactor, samples were annealed at ∼ 650 ° C in H 2 for 10 min to remove surface contamination from the nanoparticles and enhance contact to the Si substrates, allowing the NWs to grow along continuations of crystallographic directions of the substrates. 20-nm-diameter Au catalyst nanoparticles were deposited by dispersing Au aqueous suspension onto n + ( 100 ) and p + ( 111 ) Si substrates. The substrates were first cleaned, ending with 5 % HF / H 2 O (followed by minimal de-ionized water rinsing) to remove the surface oxide layer and obtain H-termination. The samples were grown by chemical vapor deposition of Ge using pre-formed Au catalyst nanoparticles on single crystal silicon, as previously reported in more detail in Ref. Here, we present a study of carrier transport in Ge NW / Si substrate heterostructures with different ( p + and n +) types of Si substrate doping. 15–19 However, very little is known about carrier transport and electrical properties of lattice-mismatched Ge NW / Si substrate heterojunctions, which are critically important for future device applications. 13,14 Optical studies show abrupt interfaces with a very thin GeSi transition layer, most likely due to the high growth rate and relatively low temperature of the vapor-liquid-solid (VLS) growth. 11,12 Recent experimental results show that Ge NW / c -Si-substrate heterojunctions with high structural quality can be obtained despite the 4.2 % Ge / Si lattice mismatch.
In contrast to conventional thin film heterostructures, where lattice mismatched heterostructure growth described by the Matthews-Blakeslee theory 9,10 often results in the formation of structural defects at the heterointerface, the small NW diameter allows efficient lateral structural relaxation without dislocation formation. 4,5 While connection of Si NWs to an integrated Si structure has been demonstrated, 6–8 little information is available about the electrical connection of lattice-mismatched semiconductor NWs (e.g., Ge) into an integrated Si platform. Catalyzed nanowires (NWs) have been grown with heterojunctions between different semiconductor materials along their length, 1–3 but less attention has been paid to the connection between the grown NW and the underlying substrate.
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