Nanoscale electrical property studies of individual GeSi quantum rings by conductive scanning probe microscopy
© Lv et al.; licensee Springer. 2012
Received: 11 October 2012
Accepted: 21 November 2012
Published: 29 November 2012
The nanoscale electrical properties of individual self-assembled GeSi quantum rings (QRs) were studied by scanning probe microscopy-based techniques. The surface potential distributions of individual GeSi QRs are obtained by scanning Kelvin microscopy (SKM). Ring-shaped work function distributions are observed, presenting that the QRs' rim has a larger work function than the QRs' central hole. By combining the SKM results with those obtained by conductive atomic force microscopy and scanning capacitance microscopy, the correlations between the surface potential, conductance, and carrier density distributions are revealed, and a possible interpretation for the QRs' conductance distributions is suggested.
KeywordsGeSi quantum rings (QRs) electrical properties SKM CAFM SCM
Self-assembled semiconductor quantum rings (QRs) are an alternative type of quantum structures, which have received great interests in recent years for their unique properties and potential applications in nano-electronic devices [1–4]. It has been reported that the QRs have especially interesting characteristics due to the ring-shaped geometry, such as persistent current and A-B effects . However, compared to the intensive theoretical studies on the ideal or lithographed QRs, the studies addressing the self-assembled QRs are relatively lacked. Furthermore, among the existing studies on self-assembled QRs, most of them are dealing with the growth mechanisms or electronic properties of QRs [6–11], while some of the studies are performed on the QRs' composition and strain distributions or atomic structures [12–16]. The electrical characteristics of the QRs, which are of vital importance to nano-electronic applications, have much less been concerned.
In recent decades, scanning probe microscopy (SPM)-based techniques have become effective means to investigate the electrical properties of individual quantum structures, which can provide non-averaged quantum properties [17, 18]. For example, conductive atomic force microscopy (CAFM) enables us to investigate the conductive properties of individual quantum structures [19–21], while scanning Kelvin microscopy (SKM)  and scanning capacitance microscopy (SCM)  are valuable tools for measuring the surface potential and carrier density distributions of individual quantum structures. These techniques have already been performed to study the electrical properties of individual quantum dots [24–31], but the electrical property studies on individual QRs are still lacking. Up to now, there are only a few papers reported about the QRs' conductance distributions [32, 33]. To gain further insight into the QRs' electrical properties as well as the intrinsic mechanisms, herein SKM is employed to investigate the surface potential distributions of individual GeSi QRs. Ring-shaped surface potential distributions of GeSi QRs are obtained, for the first time to our knowledge. By combining with the results obtained by CAFM and SCM, the correlations between the surface potential, conductance, and carrier density distributions are revealed, and a possible explanation for the QRs' conductance distribution is suggested.
The GeSi QRs studied here were grown on a p-type Si(001) wafer (1 ~ 10 Ω cm) in a solid source molecular beam epitaxy (Riber EVA-32, France) system . First, a 64-nm-thick Si buffer layer was deposited on the wafer at 640°C, followed by a 2.5-nm-thick Ge layer depositing at the same temperature. Then, a 3.2-nm-thick Si cap layer was deposited at the same temperature of 640°C to form GeSi QRs. The growth details as well as the formation mechanism of the QRs were discussed in our previous paper .
The electrical property measurements were performed on a Multimode V (Bruker Nano Surfaces, MA, USA) SPM instrument. The conductive properties of single QRs are measured by CAFM in contact mode under a DC bias. Pt-coated Si tips are applied in CAFM measurement, and a bias of −1 V is applied to the sample, while the tip is grounded. For SKM measurements, a voltage consisting of a DC bias with a small AC modulation is applied to the tip. During the scan, the DC bias is adjusted to be always equal to the contact potential difference (CPD) between the tip and the sample surface at each point through the feedback system, which forms the CPD image. SCM measures the capacitance variation with a small voltage variation, i.e., dC/dV. For a metal tip/oxide/semiconductor system, the dC/dV amplitude is inversely proportional to the carrier density in semiconductor. Thus, the carrier density distribution can be obtained from the dC/dV amplitude image. Their detailed operation principles can be found in previous reviews [17, 18, 22]. W2C-coated Si tips are employed in SKM and SCM measurements. Before CAFM and SKM measurements, the samples would be etched in diluted HF solution for 30 s to remove the native oxide layer which could keep the sample surface free-oxidized in the next 2-h when the experiments were performed in a flowing nitrogen atmosphere , and the measurements were mostly carried out during this time period. To exclude the influence of composition distribution, the sample was etched in a BPA solution (HF/H2O2/CH3COOH = 1:2:3) to remove the GeSi alloys with Ge ratio more than 6% [35, 36], leaving a Si-dominated surface.
Results and discussions
The reason why the QRs' topographic shape has an important impact on their electrical properties is not clear yet. For GeSi QDs, it is known that the QD structure can confine holes due to its composition variation. But for QRs, the holes should be confined to the whole QR structure, especially to the QRs' center due to it owns high-Ge content compared to the ring and wetting layer, if only considering the composition variation. But in our experiments, no current and lowest carrier density was measured at the QRs' center. Thus, the topographic factor should influence the carrier density distribution, though its origin is not understood yet. In previous theoretical studies dealing with ideal QRs  or InAs/GaAs QRs [5, 40, 41], it was found that either ring-shaped adiabatic potential  or carrier probability density [10, 40, 41] was achieved by considering the QRs' geometrical parameters, as well as strain , composition gradient , or piezoelectric potential [5, 41], etc. Though, for a realistic QR, the distribution of carrier density would be much complex as many factors should be taken into consideration, the main feature of the ring-shaped potential and carrier density could be expected. Since no reference has been found to report the electronic properties of GeSi QRs and the above considerations on InAs QRs are consistent with our SKM and SCM results, we suggest the QRs' ring-shaped surface potential and carrier density distributions are attributed to their geometrical shapes follows the above concept. The correlation between the conductance distribution and the carrier density distribution is direct, i.e., higher carrier density resulting in larger conductivity; our SCM results did agree with the QRs' conductance distribution. The correlation between the conductance distribution and the surface potential distribution could be described with the viewpoint of electron barrier height at the interface between the tip and the measured surface, which was introduced by Lochthofen et al. to interpret the higher conductivity of the V-defects in GaN film . The schematic band diagrams of the interfaces between the PtIr tip and the wetting layer, the rim of the QR, and the central hole of the QR are shown in Figure 4a,b,c, respectively. Since in our CAFM experiments the sample is −1 V biased with respect to the grounded tip, the electrons flow from the sample to the tip. From the results of SKM, the electron barrier height for the QRs' rim (ΦB,Rim) is found to be lower than that for the QRs' central hole (ΦB,Center) and the wetting layer (ΦB,WL). This is consistent with the CAFM results which show that the QRs' rim is more conductive than the QRs' central hole and wetting layer.
Based on the above considerations, a possible explanation of the QRs' electrical properties was suggested: the ring-shaped geometry determined that the QRs' rim has a lower barrier height with the tip and a higher carrier density, resulting in a higher conductivity at the rim, compared to the central hole and the wetting layer.
In summary, the electrical properties of individual GeSi QRs were investigated by SKM, CAFM, and SCM. Ring-shaped surface potential, conductance, and carrier density distributions are achieved on individual original and BPA-etched GeSi QRs. Based on these results, it can be suggested that the ring-shaped surface potential distribution and/or carrier density distribution are the important contributors to QRs' conductance distribution.
This work was supported by the Major State Basic Research Project of China (no. 2011CB925601), National Natural Science Foundation of China (no. 11274072), and Natural Science Foundation of Shanghai (no. 12ZR1401300).
- Huang G, Guo W, Bhattacharya P, Ariyawansa G, Perera AGU: A quantum ring terahertz detector with resonant tunnel barriers. Appl Phys Lett 2009, 94: 101115. 10.1063/1.3100407View Article
- Wu J, Li Z, Shao D, Manasreh MO, Kunets VP, Wang ZM, Salamo GJ, Weaver BD: Multicolor photodetector based on GaAs quantum rings grown by droplet epitaxy. Appl Phys Lett 2009, 94: 171102. 10.1063/1.3126644View Article
- Yu LW, Chen KJ, Song J, Xu J, Li W, Li HM, Wang M, Li XF, Huang XF: Self-assembled Si quantum-ring structures on a Si substrate by plasma-enhanced chemical vapor deposition based on a growth-etching competition mechanism. Adv Mater 2009, 19: 1577–1581.View Article
- Wu J, Wang ZM, Dorogan VG, Li S, Zhou Z, Li H, Lee J, Kim ES, Mazur YI, Salamo GJ: Strain-free ring-shaped nanostructures by droplet epitaxy for photovoltaic application. Appl Phys Lett 2012, 101: 043904. 10.1063/1.4738996View Article
- Kleemans NAJM, Bominaar-Silkens IMA, Fomin VM, Gladilin VN, Granados D, Taboada AG, García JM, Offermans P, Zeitler U, Christianen PCM, Maan JC, Devreese JT, Koenraad PM: Oscillatory persistent currents in self-assembled quantum rings. Phys Rev Lett 2007, 99: 146808.View Article
- Cui J, He Q, Jiang XM, Fan YL, Yang XJ, Xue F, Jiang ZM: Self-assembled SiGe quantum rings grown on Si(001) by molecular beam epitaxy. Appl Phys Lett 2003, 83: 2907. 10.1063/1.1616992View Article
- Lee SW, Chen LJ, Chen PS, Tsai MJ, Liu CW, Chien TY, Chia CT: Self-assembled nanorings in Si-capped Ge quantum dots on (001) Si. Appl Phys Lett 2003, 83: 5283. 10.1063/1.1635073View Article
- Baranwal V, Biasiol G, Heun S, Locatelli A, Mentes TO, Orti MN, Sorba L: Kinetics of the evolution of InAs/GaAs quantum dots to quantum rings: a combined X-ray, atomic force microscopy, and photoluminescence study. Phys Rev B 2009, 80: 155328.View Article
- Timm R, Eisele H, Lenz A, Ivanova L, Balakrishnan G, Huffaker DL, Dähne M: Self-organized formation of GaSb/GaAs quantum rings. Phys Rev Lett 2008, 101: 256101.View Article
- Okunishi T, Ohtsuka Y, Muraguchi M, Takeda K: Interstate interference of electron wave packet tunneling through a quantum ring. Phys Rev B 2007, 75: 245314.View Article
- Lei W, Notthoff C, Lorke A, Reuter D, Wieck AD: Electronic structure of self-assembled InGaAs/GaAs quantum rings studied by capacitance-voltage spectroscopy. Appl Phys Lett 2012, 96: 033111.View Article
- Stoffel M, Malachias A, Rastelli A, Metzger TH, Schmidt OG: Composition and strain in SiGe/Si(001) “nanorings” revealed by combined X-ray and selective wet chemical etching methods. Appl Phys Lett 2009, 94: 253114. 10.1063/1.3152269View Article
- Biasiol G, Magri R, Heun S, Locatelli A, Mentes TO, Sorba L: Surface compositional mapping of self-assembled InAs/GaAs quantum rings. J Cryst Growth 2009, 311: 1764. 10.1016/j.jcrysgro.2008.09.198View Article
- Biasiol G, Heun S: Compositional mapping of semiconductor quantum dots and rings. Phys Rep 2011, 500: 117–173. 10.1016/j.physrep.2010.12.001View Article
- Offermans P, Koenraad PM, Wolter JH, Granados D, Garcia JM, Fomin VM, Gladilin VN, Devreese JT: Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs. Appl Phys Lett 2005, 87: 131902. 10.1063/1.2058212View Article
- Sztucki M, Metzger TH, Chamard V, Hesse A, Holy V: Investigation of shape, strain, and interdiffusion in InGaAs quantum rings using grazing incidence X-ray diffraction. J Appl Phys 2006, 99: 033519. 10.1063/1.2170401View Article
- Oliver RA: Advances in AFM for the electrical characterization of semiconductors. Rep Prog Phys 2008, 71: 076501. 10.1088/0034-4885/71/7/076501View Article
- Avila A, Bhushan B: Electrical measurement techniques in atomic force microscopy. Crit Rev Solid State Mat Sci 2010, 35: 38. 10.1080/10408430903362230View Article
- Li C, Bando Y, Golberg D: Current imaging and electromigration-induced splitting of GaN nanowires as revealed by conductive atomic force microscopy. ACS Nano 2010, 4: 2422. 10.1021/nn100223jView Article
- Xu M, Pathak Y, Fujita1 D, Ringor C, Miyazawa K: Covered conduction of individual C60 nanowhiskers. Nanotechnology 2008, 19: 075712. 10.1088/0957-4484/19/7/075712View Article
- Zhao SH, Lv Y, Yang XJ: Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy. Nanoscale Res Lett 2011, 6: 498. 10.1186/1556-276X-6-498View Article
- Melitz W, Shena J, Kummel AC, Lee S: Kelvin probe force microscopy and its application. Surf Sci Rep 2011, 66: 1–27. 10.1016/j.surfrep.2010.10.001View Article
- Eckhardt C, Silvano de Sousa J, Brezna W, Bethge O, Bertagnolli E, Smoliner J: Frequency dependent capacitance spectroscopy using conductive diamond tips on GaAs/Al2O3 junctions. J Appl Phys 2010, 107: 064320. 10.1063/1.3354030View Article
- Tanaka I, Kamiya I, Sakaki H, Qureshi N, Allen SJ, Petroff PM: Imaging and probing electronic properties of self-assembled InAs quantum dots by atomic force microscopy with conductive tip. Appl Phys Lett 1999, 74: 844. 10.1063/1.123402View Article
- Xue F, Qin J, Cui J, Fan YL, Jiang ZM, Yang XJ: Studying the lateral composition in Ge quantum dots on Si(001) by conductive atomic force microscopy. Surf Sci 2005, 592: 65. 10.1016/j.susc.2005.06.082View Article
- Shusterman S, Raizman A, Sher A, Paltiel Y, Schwarzman A, Lepkifker E, Rosenwaks Y: Nanoscale mapping of strain and composition in quantum dots using Kelvin probe force microscopy. Nano Lett 2089, 2007: 7.
- Yamauch I, Tabuchi M, Nakamura A: Size dependence of the work function in InAs quantum dots on GaAs(001) as studied by Kelvin force probe microscopy. Appl Phys Lett 2004, 84: 3834. 10.1063/1.1745110View Article
- Smoliner J, Brezna W, Klang P, Andrews AM, Strasser G: Quantitative scanning capacitance microscopy on single subsurface InAs quantum dots. Appl Phys Lett 2008, 92: 092112. 10.1063/1.2885087View Article
- Smaali K, Hdiy AE, Molinari M, Troyon M: Band-gap determination of the native oxide capping quantum dots by use of different kinds of conductive AFM probes: example of InAs/GaAs quantum dots. IEEE Trans Electron Devices 2010, 57: 1455.View Article
- Wu R, Zhang SL, Lin JH, Jiang ZM, Yang XJ: Bias-dependent conductive characteristics of individual GeSi quantum dots studied by conductive atomic force microscopy. Nanotechnology 2011, 22: 095708. 10.1088/0957-4484/22/9/095708View Article
- Zhang YF, Ye FF, Lin JH, Jiang ZM, Yang XJ: Increased conductance of individual self-assembled GeSi quantum dots by inter-dot coupling studied by conductive atomic force microscopy. Nanoscale Res Lett 2012, 7: 278. 10.1186/1556-276X-7-278View Article
- Mlakar T, Biasiol G, Heun S, Sorba L, Vijaykumar T, Kulkarni GU, Spreafico V, Prato S: Conductive atomic force microscopy of InAs/GaAs quantum rings. Appl Phys Lett 2008, 92: 192105. 10.1063/1.2928220View Article
- Zhang SL, Lv Y, Jiang ZM, Yang XJ: Electrical properties of individual self-assembled GeSi quantum rings. J Appl Phys 2011, 110: 094313. 10.1063/1.3658816View Article
- Wu R, Li FH, Jiang ZM, Yang XJ: Effects of a native oxide layer on the conductive atomic force microscopy measurements of self-assembled Ge quantum dots. Nanotechnology 2006, 17: 5111. 10.1088/0957-4484/17/20/012View Article
- Carns TK, Tanner MO, Wang KL: Chemical etching of Si1-x Gex in HF:H202:CH3COOH. J Electrochem Soc 1995, 142: 1260–1266. 10.1149/1.2044161View Article
- Holläder B, Buca D, Mantl S, Hartmannb JM: Wet chemical etching of Si, Si1−xGex, and Ge in HF:H2O2:CH3COOH. J Electrochem Soc 2010, 157: H643-H646. 10.1149/1.3382944View Article
- Melitz W, Shen J, Lee S, Lee JS, Kummel AC, Droopad R, Yu ET: Scanning tunneling spectroscopy and Kelvin probe force microscopy investigation of Fermi energy level pinning mechanism on InAs and InGaAs clean surfaces. J Appl Phys 2010, 108: 023711. 10.1063/1.3462440View Article
- Lee NJ, Yoo JW, Choi YJ, Kang CJ, Jeon DY, Kim DC, Seo S, Chung HJ: The interlayer screening effect of graphene sheets investigated by Kelvin probe force microscopy. Appl Phys Lett 2009, 95: 222107. 10.1063/1.3269597View Article
- Baumgart C, Helm M, Schmidt H: Quantitative dopant profiling in semiconductors: a Kelvin probe force microscopy model. Phys Rev B 2009, 80: 085305.View Article
- Barker JA, Warburton RJ, O'Reilly EP: Electron and hole wave functions in self-assembled quantum rings. Phys Rev B 2004, 69: 035327.View Article
- Filikhin I, Suslov VM, Vlahovic B: Electron spectral properties of the InAs/GaAs quantum ring. Physica E 2006, 33: 349. 10.1016/j.physe.2006.04.013View Article
- Lochthofen A, Mertin W, Bacher G, Hoeppel L, Bader S, Off J, Hahn B: Electrical investigation of V-defects in GaN using Kelvin probe and conductive atomic force microscopy. Appl Phys Lett 2008, 93: 022107. 10.1063/1.2953081View Article
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