A cornerstone in the successful application of semiconductor nanowire devices is controlled impurity doping. In this review article, we discuss the key results in the field of semiconductor nanowire doping. Considerable development has recently taken place in this field, and half of the references in this review are less than 3 years old. We present a simple model for dopant incorporation during in situ doping of particle-assisted growth of nanowires. The effects of doping on nanowire growth are thoroughly discussed since many investigators have seen much stronger and more complex effects than those observed in thin-film growth. We also give an overview of methods of characterizing doping in nanowires since these in many ways define the boundaries of our current understanding.

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Doping of semiconductor nanowires

We report quantitative, noninvasive and nanoscale-resolved mapping of the free-carrier distribution in InP nanowires with doping modulation along the axial and radial directions, by employing infrared near-field nanoscopy. Owing to the technique's capability of subsurface probing, we provide direct experimental evidence that dopants in interior nanowire shells effectively contribute to the local free-carrier concentration. The high sensitivity of s-SNOM also allows us to directly visualize nanoscale variations in the free-carrier concentration of wires as thin as 20 nm, which we attribute to local growth defects. Our results open interesting avenues for studying local conductivity in complex nanowire heterostructures, which could be further enhanced by near-field infrared nanotomography.

Nanoscale Free-Carrier Profiling of Individual Semiconductor Nanowires by Infrared Near-Field Nanoscopy

Axial GaAs nanowire p–n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor–liquid–solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p–n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p–n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor–liquid–solid mechanism. Surface passivation using ammonium s...

Direct Determination of Minority Carrier Diffusion Lengths at Axial GaAs Nanowire p–n Junctions

We studied the local voltage drop in functionalized graphene sheets of subμm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current–voltage characteristics and an intrinsic conductivity of about 3.7 × 105 S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be <6.3 × 10–7 Ωcm2.

Local Voltage Drop in a Single Functionalized Graphene Sheet Characterized by Kelvin Probe Force Microscopy

Physical properties at the nanoscale are novel and different from those in bulk materials. Over the last few decades, there has been an ever growing interest in the fabrication of nanowire structures for a wide variety of applications including energy generation purposes. Nevertheless, the study of their transport properties, such as thermal conductivity, electrical conductivity or Seebeck coefficient, remains an experimental challenge. For instance, in the particular case of nanostructured thermoelectrics, theoretical calculations have shown that nanowires offer a promising way of enhancing the hitherto low efficiency of these materials in the conversion of temperature differences into electricity. Therefore, within the thermoelectrical community there has been a great experimental effort in the measurement of these quantities in actual nanowires. The measurements of these properties at the nanoscale are also of interest in fields other than energy, such as electrical components for microchips, field effect transistors, sensors, and other low scale devices. For all these applications, knowing the transport properties is mandatory. This review deals with the latest techniques developed to perform the measurement of these transport properties in nanowires. A thorough overview of the most important and modern techniques used for the characterization of different kinds of nanowires will be shown.

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Review on measurement techniques of transport properties of nanowires

We demonstrate the potential of Kelvin probe force microscopy for simultaneously probing the topography and the work function of individual nanowires. Our technique allows us to visualize both the material and the doping contrast in single GaAs-based nanowires without the need to electrically contact the nanowires. In a GaAs/GaP heterostructure nanowire, a core-shell structure is found. This is attributed to a thermally activated radial overgrowth of GaAs, while in the GaP region the vertical nanowire growth dominates. In partially p-doped GaAs nanowires the doping transitions can be localized and the width of the depletion layer is estimated.

https://iopscience.iop.org/article/10.1088/0957-4484/20/38/385702/pdf

Material and doping transitions in single GaAs-based nanowires probed by Kelvin probe force microscopy

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm2, which is important for potential applications in concentrator solar cells.

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

We report on the electrical characterization of a single p‐doped GaAs nanowire by contactless Kelvin Probe Force Microscopy. By measuring the local voltage drop along the nanowire quantitatively, a resistance of the nanowire of 57 kΩ is extracted and taking the wire geometry into account, an effective carrier concentration of 6⋅1017 cm−3 can be deduced.

Local Electrical Analysis of a Single Semiconductor Nanowire by Kelvin Probe Force Microscopy

We have studied the local surface potential and the voltage drop along individual VLS grown GaAs nanowires using Kelvin probe force microscopy. With the obtained information, we identify a core–shell structure in GaAs/GaP heterostructure nanowires, which we attribute to the difference in radial and vertical growth between the two semiconductor materials. In p-doped GaAs nanowires, qualitative and quantitative doping levels are estimated. Furthermore, we find a better incorporation of the zinc compared to the carbon to realize doping in partially p-doped GaAs nanowires by localizing the doping transitions and estimating the width of their depletion layers. Additionally, the p–n junction can be localized with a resolution better than 50 nm and the bias dependence of the depletion layer width can be studied.

Sasa Vinaji

Papers

Material and doping transitions in single GaAs-based nanowires probed by Kelvin probe force microscopy

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

Local Electrical Analysis of a Single Semiconductor Nanowire by Kelvin Probe Force Microscopy

Material and Doping Contrast in III/V Nanowires Probed by Kelvin Probe Force Microscopy