Morphology And Electrical Properties Of The Conductive Atomic Force Microscopy Study Of Silicon-germanium Quantum Dots And Rings

Semiconductor quantum dots(QDs) have received considerable attention because of their potential applications in photoelectron,micro-electronics and single electron devices.Self-assemble GeSi QDs have one of major advantages of its compatibility with conventional Si integrated-circuit technology.The quantum properties of quantum dots are essentially important to the application of most nano or single electron devices.Conductive atomic force microscopy(CAFM) is a promising technique capable of simultaneous topographic and conductance measurements over nano-contact area.Microstructure and physical properties of individual QD obtained by CAFM will be useful for controlling growth and application of QDs.Under the above background,this thesis is focused on topographic and conductance properties of individual QDs and quantum rings(QRs) and includes the following four parts:The first part is investigating the effects of native oxide layer on the CAFM measurements of GeSi QDs.After exposed in ambient,the current-voltage curves of the quantum dots can be well fitted by Fowler Nordheim tunneling model due to the inevitable native oxide layer,whereas Schottky emission model can be explained theⅠ-Ⅴcharacteristics well after the oxide layer was removed by HF etching.Ⅰ-Ⅴcharacteristics were also affected by the applied normal force,since it can change the effective oxide layer thickness.However,despite of the strong effect of native oxide layer on the conductive properties,the current distributions of individual QDs remain almost the same as the oxide layer thickness changed by HF etching or different normal forces applied.Thus our results provide important evidence to the application of CAFM analysis technique in ambient.The second part is the studing the effects of applied bias on the current distribution of invidual GeSi QDs.The current distribution of QDs holds ring-like characteristics under -0.5～-2.0 bias and the statistic of current magnitude can be well fitted by log-normal distribution.The relation of histogram peak position and applied bias in this field shows a good F-N tunneling fitting.The current distribution of QDs shows disk-like characteristics under -2.5～-4.0 bias and central part of QD is high conductive.The statistic of current magnitude can be well fitted by multivariant log-normal distribution.The relation of first histogram peak position and applied bias can be explained by thermal electron emission model.In addition the current magnitude is saturated at high bias.Therefore current mechanisms are affected by the applied bias.The third part is probing transient hole trapping phenomenon in individual GeSi QDs.By controlling the bias voltage sweep in certain fast sweep rate range,a novel current peak is observed in the current-voltage characteristics measured on individual GeSi quantum dots.The current peaks are detected only during the backward voltage sweep immediately after a forward sweep.The current peak position and intensity are found to depend strongly on the voltage sweep conditions.This kind of fast sweep current-voltage characteristic is very different from the ordinary steady state current behaviors of quantum dots measured previously.The origin of this phenomenon is attributed to the transient carrier trapping in the potential well formed by the quantum dot sandwiched between the top oxide layer and bottom Si substrate.The fourth part is GeSi QRs electrical property studies.The current distribution change during the transition from dome QD,pyramid QD,mound to QRs was detected by CAFM,which changes from ring-like,crisscross,disk-like to ring-like. The ring-like current characteristics of QRs are found not to depend on the applied bias.When it comes toⅠ-Ⅴcharacteristic on QRs,negative difference resistance phenomenon is observed.It is attributed to strain-induced-potential for hole resonant tunneling of planar QR structure.