Deliverable D2.3 - LTH · method to use (contactless ... This means that we can determine if there are any differences along a single nanowire. Spatially resolved four-probe resistivity

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

Deliverable D2.3:

“Report on studies of doping of the structures”

Responsible author: Dr Anders Gustafsson, ULUND

Summary

The effect of doping has been studied for a number of material systems by ULUND and DTU. We have applied a number of techniques to study the relative incorporation as well as extracting concentrations. The main findings are:

• Hall and resistivity measurements. We have applied Hall and resistivity investigations of p-type GaP and n-type GaN cores. Both types of measurements give values of the doping concentrations, wehre the more complex Hall measurements give a higher precission.

• Combined Optical and Hall investigations. As the Hall investigation are technologically challenging, we have developed a method to use (contactless) optical studies to extract the concentrations. This has proved to be very accurate for the InP shells taht we have concentrated on so far.

• Optical characterization of Mg-doped nanowires. We have investigated the crucial Mg-doping of the GaN shells of the GaN nanowires. The Mg doping behaves as expected. However, we have identified a potential Si contamination in the shell, leading to a reduced hole concentration. The incorporation of Mg can under certain conditions be inhomogeneous alonßg the nanowire.

• Off-axis electron holography. We determined the built-in potential of a series of core-shell GaP nanowires. From this study, we conclude that Sn incorporates more efficiently as an n-type dopant in GaP NWs. The n-type doping of the shell was not successful with Si, and was only achieved with high molar fraction of Se.

• Syncrotron-based studies of doping.

During the first 24 months of the project we have developed and used techniques for dopant studies of semiconductor nanowires (NWs). We have investigated different NWs using both Scanning Tunneling Microscopy (STM) / Spectroscopy (STS) and Scanning Gate Microscopy (SGM).



2

Electrical characterization

We have characterized the doping and charge transport properties in both p-type GaP and n-type GaN single nanowires using electrical measurement techniques. In this section, we present data on single cores, but we have also studied radial layers in core / shell pn-junctions. Our flagship electrical measurement technique is the spatially resolved Hall effect measurement [1] [2], but also spatially resolved four-probe resistivity and back-gate field-effect measurements have been carried out. The combination of Hall effect and resistivity measurements on a single nanowire is a powerful tool to evaluate both doping level and carrier mobility in different segments of a nanowire.

In order to carry out Hall effect measurements on a single nanowire, a high precision contacting scheme is used. The nanowires are transferred to an insulating substrate and mask patterns for contacts are defined using electron beam lithography. Metal contacts are then evaporated onto the sample. Two finished devices are shown in figure 1. Using this device design it is possible to conduct Hall effect measurements, four-probe resistivity measurements and back-gate field-effect measurements on the same device.

Figure 1: Scanning electron micrographs of single nanowire devices (GaP left, GaN right) for Hall effect measurements. The wires are in the middle of the images with a contact at each end. These leads are used for passing the current through the wires in order to generate a Hall voltage. In addition, there are three pairs of contacts to the sides of the wires for measuring the Hall voltage. Three pairs give us access to three different positions on the wire. This means that we can determine if there are any differences along a single nanowire.

Spatially resolved four-probe resistivity measurements have also been used to investigate nanowires with different doping parameters. The resistivity gives a good estimate of the doping concentration and homogeneity of the nanowires and can be used for example to compare and evaluate different growth conditions and post-growth treatments in cases where Hall measurements are hard to implement. Examples of comparisons between nanowires grown with different parameters can be seen in figure 2.



3

Figure 2: Box plots showing the resistivity of nanowires with different growth parameters. Left: The plot shows GaP cores with different doping concentrations. Right: shows GaN cores, also with different doping concentrations.

Hall effect measurements have successfully been carried out on n-type GaN nanowires (see figure 3) and much work has been put into improving the technique and applying it to new materials and device geometries. The two largest challenges are placing electrical contacts with sufficient precision (the positioning needs to be accurate down to about 20 nm) and to accurately measure the small voltages associated with the Hall effect in a nanowire. Currently we are building a new, improved system for Hall effect measurements. This setup will feature a new stronger magnet, increasing the Hall voltage 10 times, noise shielding and advanced electrical measurement instruments. It will also be highly automated in order to enable collection of large data series accurately and efficiently. To extract the doping concentration from the Hall effect measurements, finite element method simulations are used (figure 3). Finite element simulations have also been used to simulate contact properties and surface depletion in the nanowires.

Figure 3: Left: Finite element simulation showing the Hall voltage in a GaN nanowire. Right: Measured Hall voltage in a GaN nanowire as a function of magnetic field. The error bars show the standard deviation of 100 subsequent voltage measurements.



4

Combination of Hall measurements and optical characterization

The Hall measurements are very time consuming, especially for the positioning the contacts on the wires. It is therefore important to device another simpler scheme to determine the dopant concentration. One such scheme is to study the shape and position of the emission peak of the luminescence. There are many factors that influence the shape of the emission from the nanowires. For low doping concentrations, the peak position experiences a minor red shift with increasing doping concentration This is caused by the bandgap narrowing resulting from the increased carrier concentration. As the concentration increases, the emission starts to blue-shift. At the same time the peak width increases. This increase is caused by an increasing disorder of the distribution of the dopants. When the concentration increases to a level where the Fermi-level rises above the conduction band edge, the emission peak broaden further, showing a high-energy tail that reflects the tail of the Fermi-distribution of the carriers. This analysis is illustrated in Figure 4 for a series of core-shell InP nanowires. They were all grown on the same p-type core, but with a varying concentration of n-type dopants, ranging from nominally un-doped to highly n-doped, in the shell. For this series, the wires were first contacted, like in figure 1, and the carrier concentrations were determined by Hall measurements. After they were contacted, the same contacted wires were studied by photoluminescence. In order to make proper correlations between the two techniques, the luminescence, as well as the Hall measurements were performed at room temperature. For this analysis, the framework of the extraction of the concentration from the shape and position of the luminescence from the wires has been developed. We are now at a point where we can determine the dopant concentrations from the luminescence spectra.

Figure 4. Illustrations of how the luminescence can be used to determine the doping concentration in InP shells in core-shell nanowires. Left: With increasing doping concentration (denoted 0 to 70 in the image) there is a gradual broadening of the emission peak and a simultaneous blue-shift of the peak position. Centre: For higher doping concentrations, only the high-energy flank blue-shifts. Right: When compared to the Hall data on the same wires, there is a good agreement between the two.



5

Optical characterization of Mg-doped GaN nanowires

The GaN nanowire structures are typically grown using a template consisting of a substrate (silicon or sapphire) overgrown with a thin n-type nitride layer, followed by an insulating mask layer (SiO2 of Si4N3). Holes of a suitable diameter (of the order 100 nm) are prepared in the mask with nano imprint lithography (NIL), the nanowires are then nucleated at the bottom of the holes and grown to desired length. We have discovered that the use of Si4N3 mask appears to induce unwanted Si doping in the structures. This is illustrated in figure 5 with different PL spectra corresponding to a different degree of Si contamination of samples consisting of an Mg-doped radial layer surrounding an n-type Si-doped core. The Si-concentrations have been estimated comparison with a set of Si-doped planar GaN layers grown on bulk GaN substrates. For these planar layers the Si-doping affects the linewidth of the main near bandgap donor bound exciton peak shows an approximately linear increase from 1 meV at [Si] = 1017 cm-3, to about 10 meV at [Si] = 1018 cm-3, and so on. At the highest doping level (green curve) the concentration is above the Mott limit, and the broad spectrum corresponds to electrons in the conduction band recombining with holes in the valence band. It is concluded that for the sample with the green PL curve the Si concentration is in the mid at [Si] = 1018 cm-3 range, causing a severe compensation of the intended Mg-doping. This leads to a reduction the hole concentration of the p-type layer.

Figure 5. Near bandgap photoluminescence (PL) spectra at 4 K of Mg-doped GaN nanowires grown by MOCVD under different growth conditions, inducing parasitic Si doping compensating the Mg acceptors. The different curves indicate different estimated Si concentrations in the nanowires: black - 6x1017 cm-3, blue – 1x1018 cm-3 green - high 1018 cm-3. The red curve is the PL for an undoped planar GaN layer grown on a bulk GaN template.



6

PL spectra for the Mg-doped nanowire samples were compared with Mg-doped GaN control samples grown on sapphire (i.e. without any Si4N3mask). Figure 6 shows the PL spectrum of such a sample at 4 K. It shows a near bandgap spectrum dominated by Mg-related peaks, with a minor contribution only from the Si donors. This is what is expected from Mg-doping in the 1019 cm-3 domain when grown by MOCVD (see e.g. ref. 3)

Our conclusion is that the Si4N3mask is affected by the growth atmosphere so that Si is released and incorporated into the nanowire during growth. Further investigations are under way to minimize this effect.

Figure 6. Near bandgap PL spectra at 4 K of a planar Mg-doped GaN layer on sapphire, before and after Mg activation. The two dominant peaks are related to Mg acceptors, the Si donor peak at the highest energy is quite weak.



7

Spatially resolved luminescence studies

The optical studies of the dopant concentration of the nanowires using photoluminescence are often sufficient. However, when there are significant gradients in the concentration along a single nanowire, we need to use techniques with a higher spatial resolution than our photoluminescence set-up. Even with our photoluminescence set-up enhanced by a solid immersion lens, the spatial resolution is still limited to about 500 nm. In these cases, we use the technique of cathodoluminescence is a scanning electron microscope. Under favourable conditions, we can achieve a spatial resolution of about 50 nm. Here, we illustrate the approach with a study of the Mg incorporation in a radial GaN layer. In most cases, the intensity of the emission related to the dopants increases with the concentration. It is therefore only necessary to record images of the emission in order to get a map of the dopant distribution. In some cases, it is not as straight forward. This is typically the case for Mg in GaN. The intensity drops with concentration and at the same time the shape of the emission changes. It goes from a series of well-defined peaks (one peak with a series of phonon replicas) to a broad peak. This peak is also red-shifted. This is illustrated in figure 7, which shows a single GaN wire containing a radial pn-junction, still attached to the substrate. One very special feature of GaN nanowires is that it is possible to cleave them along the length, which leaves the structure exposed. Each radial layer appears with two braches around the core. The figure shows that the emission the emission related to the Mg-doped outer p-layer. It is obvious that the intensity goes down along the wire, from base to tip. When studying the spectrally resolved emission, it is clear that the Mg-concentration actually goes up towards the top, despite the decrease in intensity. This trend is identified by the shape of the spectra rather that the absolute intensity.

Figure 7. Left: A monochromatic cathodoluminescence image, with the intensity in red overlaid on the corresponding SEM image (grey). The wire is cleaved through the length and the radial p-type layer therefore appears as two parallel lines. This image shows decreasing emission intensity from base to tip. Right: Spot mode spectra shows that there is a change in the spectral signature from base to tip. This indicates that the concentration increases from base to tip, despite the reduced intensity.



8

GaP nanowires studied by off-axis electron holography The incorporation of dopants into the GaP crystal structure was evaluated by measuring the built-in potentials across the core-shell pn-junctions using off-axis electron holography. For preparing cross-sectional TEM specimens we used FIB milling. Fifteen different core-shell pn-junction GaP nanowire samples with the same p-type core but different shells were grown by MOVPE. Three different precursors (SiH4, DTBSe and TESn) were used for n-type doping of the nanowire shell, and for each precursor the nanowire shell were doped with five different dopant concentrations using different molar fractions of the precursors.

The graphs in figure 8 show the measured built-in potentials across the core-shell junctions for the three precursors as a function of molar fractions. The most important point is that only for TESn precursor, the core-shell pn-junction was achieved for all the investigated molar fractions. When TESn was used, the measured built-in potentials increased almost exponentially (linearly when plotted in logarithmic scale) with the molar fractions, meaning that the dopant concentration increases with the TESn molar fraction without being saturated. In contrast, SiH4 did not act as an n-type precursor although SiH4 molar fractions were about three orders of magnitude higher than TESn molar fractions. Instead of a pn-junction, a pp+-junction was formed across the core-shell when SiH4 was used as precursor. Also in the case of DTBSe, a pp+-junction was formed rather than a pn-junction across the core-shell for all the DTBSe molar fractions except the highest one. Only when the high molar fraction of χDTBSe = 5.2×10-6 (about two orders of magnitude higher than that used for TESn) was used, n-type shell was achieved for DTBSe precursor. From this measurement it is clear that Sn incorporates as an active n-type dopant into the GaP NWs more efficiently than Si and Se.

For the same growth time, the n-type shell is thicker for TESn precursor than for DTBSe precursor. Comparing the n-type shell thickness of Sn-doped NWs with Se-doped NWs (inset of figure 8) reveals that the radial growth rate is larger when TESn is used as an n-type precursor. It can be concluded that the type of precursor used for n-type doping affect the growth rate. Note that apart from the type and the molar fraction of n-type precursors, all the other growth conditions, including the shell growth time, were the same for all 15 NW samples.

The appearance of Sn-rich particles on the surfaces of NWs, even at the lowest TESn molar fraction of χTESn = 5.2×10-7, and the increase of the density of these particles with TESn molar fraction, observed using scanning electron microscopy, raised concerns about the uniformity of n-type doping using TESn. However, electron holography shows that despite the presence of Sn-rich particles on the surfaces of NWs, active Sn dopants distribute uniformly within the shell of the NWs. Although Sn atoms migrate to the NW surfaces and segregate there, this does not affect the sharpness of the junction and the uniformity of the electrostatic potential distribution along the NWs shell. These findings can be seen in figure 9 where, as an example, a typical holographic phase image of a Sn-doped GaP NWs is shown. Also electrical biasing of several individual core-shell pn-junction GaP NWs with Sn-rich particles on their surfaces shows no sign of short cutting the junction by these particles. A



9

typical IV curve measured from an individual core-shell pn-junction NW is shown in figure 10.

Off-axis electron holography also showed that p-type dopants (DEZn was used as the p-type precursor) are not distributed uniformly within the core. As can be seen in figure 9, there is a cylindrical region within the core with different electrostatic potential than the rest of the core. The origin of this non-uniformity could be due to: a) the migration of Zn dopants to the surface of nanowires during the growth or b) the fact that two different growth mechanisms are responsible for the core growth (the vapour-liquid-solid (VLS) and the vapour-solid (VS) growth mechanisms). It is likely that the dopant incorporation rate for the VLS and VS mechanisms is different. Further investigations on this matter are underway.

Figure 8. Measured built-in potentials across the core-shell GaP nanowires as a function of precursor molar fraction for three different n-type precursors. The n-type doping of the shell was only successful for all the molar fractions when TESn was used. Although SiH4 molar fraction was significantly larger, no core-shell pn-junction was formed when SiH4 was used as an n-type precursor, instead, only pp+-junction was formed. Also in the case of DTBSe, the n-type doping of the shell was only achieved when high molar fraction of χDTBSe = 5.2×10-6 (about two orders of magnitude higher than that used for TESn) was introduced. The holographic phase images in the insets, shown as examples, reveal that the n-type shell thickness is larger in the Sn-doped NWs than in the Se-doped NWs. The thick black arrows in the phase images are pointing to the region of the core, which has a higher potential than the rest of the core.



10

Figure 9. a) Phase and b) amplitude image of a Sn-doped NW, and c) its corresponding electrostatic potential measurement. The electrostatic potential profile corresponds to the arrow crossing the NW in (b) from right to left. The dark thick arrows in (b) are pointing to the Sn-rich particles on the NW surface. It can be seen that the core-shell pn junction is not affected by these particles.

Figure 10. An example of current-voltage (IV) measurement from the core-shell pn-junction of a single GaP NW with Sn-rich particles present on its surface, showing that these particles do not short cut the junction. The diode characteristic of the pn-junction is clear in the IV curve.

Report on STM/SGM and PES/PEEM studies on doping of nanowires for light

During the first 24 months of the project we have developed and used techniques for dopant studies of semiconductor nanowires (NWs). We have investigated different NWs using both



11

Scanning Tunneling Microscopy (STM) / Spectroscopy (STS) and Scanning Gate Microscopy (SGM). While these techniques are in principle surface sensitive they can be used to probe “bulk” doping as well by suitable design of the experiment. Our measurements are combined with state-of-the-art synchrotron based PhotoEmission Electron Microscopy (PEEM) and Spectroscopy (PES).

Investigating doping along III-V semiconductor nanowires using STM

Until recently, STM/S studies of InP and InAs NWs were limited by the presence of defects and/or surface reconstructions, which made it difficult to reach definite conclusions especially on electronic properties inside the NW such as doping. This was too bad, since previously STM studies on embedded nanostructures (so-called cross-sectional STM) have demonstrated to provide a unique and powerful way to study the doping levels inside III-V nanostructures by determining the Fermi-level position in the material. We now managed to obtain large defect free and unreconstructed surfaces on GaAs NWs of perfectly tailored segments of both crystal structures of the NWs – Wurtzite (Wz) and Zincblende (Zb). This led the way for beautifully well-defined experiments in which the crystal structure is perfectly controlled and the experimental conditions (STM tip) will be exactly the same when comparing different parts of the nanowires. Such experiments demonstrated that we could determine the relative position of the Fermi level along a material with a precision in the range of 10 meV. As a result, we can give quantitative measures of doping levels and especially differences in doping along NWs. Since these groundbreaking measurements we have extended this concept so that we can now perform similar experiments for InP and InAs NWs. As a result, we have the means to determine the doping levels along a nanowire with nanoscale precision on most III-V materials including ternaries.

• Figure 11: a) STM image of 4 individual crystal facets of two different types of crystal symmetry taken at once. Three images to the side show individual atom resolution on 3 out of 4 facets – surfaces are perfectly crystalline. b) Local STS has revealed the quantitative band alignment and band gaps on nanowires for the first time[ref 1].



12

It can be noted that we are also sensitive to the influence of the surface Fermi level pinning (surface doping) on the NW, which can completely control NW doping properties in some case. Using STM and PES we can also study the influence of impurities at the surface on the doping and have done that especially on InP NWs with different doping levels and with differently doped segments along the nanowire.

Investigating local conductivity along III-V semiconductor nanowires using SGM

High-resolution surface characterization has until now only been realized for randomly distributed NWs. So far, it has not been possible to study the electric performance of nanowire devices and their local electronic and surface properties simultaneously. But for any device application it is crucial to know the I-V characteristics of the individual NWs and their correlation to the exact NW structure. In order to bridge this gap, we have deposited III-V semiconductor NW heterostructures on an insulating substrate (SiO2/Si), processed electrical contacts to individual NWs, and measured their local structure using Atomic Force Microscopy (AFM). By using a metallic AFM tip and applying a bias between the tip and the NW, we can apply a local gate and measure the source-drain current through the contacted NW while the gate is scanned, a technique called Scanning Gate Microscopy (SGM). With a special QPlus setup, based on a combined STM/AFM probe tip mounted on a tuning fork and operating in ultrahigh vacuum, we could reach a lateral resolution on top of the NW far beyond previous SGM imaging. This setup now allows us to measure local charge carrier mobility and therewith doping levels in a field-effect-transistor geometry with unprecedented spatial sensitivity.

Figure 12: (a) SGM image of the source-drain current through the NW (top) together with the simultaneously acquired AFM topography image (middle). A sketch of the imaged GaSb/InAs NW contacted for the SGM measurement is shown at the bottom. At the InAs segment, the proximity of the biased gate leads to a strong increase in NW current. (b) Zoom-in at the InAs part of the NW, showing again an AFM topography image (left) and the corresponding SGM current image (right). Note that individual surface steps of the NW side facets are visible in the SGM image [ref 2].

(a) (b)



13

References

[1] Storm et al., Nature Nanotechnology 7, 718–722 (2012), doi:10.1038/nnano.2012.190 [2] Heurlin et al., Nano Lett., 2014, 14 (2), pp 749–753, doi: 10.1021/nl404039d [3] B. Monemar et al., J Appl Phys 115, 053507 (2014). [4] M. Hjort, S. Lehmann, J. Knutsson, R. Timm, D. Jacobsson, E. Lundgren, K. Dick, and

A. Mikkelsen, Direct imaging of atomic scale structure and electronic properties of GaAs wurtzite and zinc blende nanowire surfaces, Nano Lett. 13, 4492 (2013)

[5] J. L. Webb, O. Persson, K. A. Dick, C. Thelander, R. Timm, and A. Mikkelsen, Scanning Gate Microscopy Measurements on InAs/GaSb Nanowire Esaki Diode Devices, Nano Res., in print (2014)

Documents

Deliverable D2.3 - LTH · method to use (contactless ... This means that we can determine if there are any differences along a single nanowire. Spatially resolved four-probe resistivity