Atom-by-Atom Analysis of Semiconductor Nanowires with Parts Per

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Atom by atom analysis of semiconductor nanowires with ppm sensitivity Sebastian Koelling, Ang Li, Alessandro Cavalli, Simone Assali, Diana Car, Sasa Gazibegovic, Erik P.A.M. Bakkers, and Paul M. Koenraad Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03109 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Atom by atom analysis of semiconductor nanowires with ppm sensitivity S. Koelling,

∗,†

A. Li,

†, ‡

A. Cavalli,

†

E.P.A.M. Bakkers,

†Eindhoven

S. Assali,

†, ¶

†

D. Car,

†, ¶

and P.M. Koenraad

S. Gazibegovic,

†, ¶

†

University of Technology, Photonics and Semiconductor Nanophysics, Eindhoven, 5600 MB

‡Beijing

University of Technology, Key Lab of Microstructure and Property of Advanced Materials, Beijing, 100024

¶Kavli

Institute, Quantum Transport Group, Delft, 2628 CJ

E-mail: [email protected]

Abstract The functionality of semiconductor devices is determined by the incorporation of dopants at concentrations down to the parts-per-million level and below. Optimization of intentional and unintentional impurity doping relies on methods to detect and map the level of impurities. Detecting such low concentrations of impurities in nanostructures is however challenging to-date as on the one hand methods used for macroscopic samples cannot be applied due to the inherent small volumes or faceted surfaces and on the other hand conventional microscopic analysis techniques are not suciently sensitive. Here, we show that we can detect and map impurities at the ppm-level in semiconductor nanowires using Atom Probe Tomography. We develop a method applicable to a wide variety of nanowires relevant for electronic and optical devices. We expect that it will contribute signicantly to the further optimization of the synthesis of nanowires, nanostructures and devices based on these structures. 1

ACS Paragon Plus Environment

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Keywords Atom Probe Tomography, Nanowires, Impurities Detecting and mapping impurities in nano-structures is challenging as they are composed of a relatively small number of atoms due to their inherently small dimensions. Controlling the impurity distribution in nanostructures used for electrical

1

and optical applications

2,3

however is crucial in order to optimize the performance of the respective devices. Such devices are typically based on nanowires grown via a bottom-up approach

4

and to-date the

growth of these nanowires is generally monitored by transmission electron microscopy (TEM) based methods like energy-dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS). These methods reach sensitivities down to 0.1 %

5

which is insucient to

detect typical doping and impurity concentrations in semiconductors. Secondary Ion Mass Spectroscopy (SIMS), as the standard method for monitoring doping proles in the semiconductor industry,

6

is unfortunately not applicable to nanowires. Therefore, Atom Probe

Tomography (APT) has been proposed as an alternative.

717

APT of nanowires has so far been hampered by the relatively complex preparation of suitable samples.

8,10,11,13

Here, we develop a technique to isolate single nanowires from a

forest of as-grown nanowires to carry out APT. It enables us to map the presence of matrix atoms and impurities with sub-nanometer spatial resolution and unprecedented sensitivity down to the ppm-level or a doping level slightly below 10

17

atoms/cm

3

respectively.

We

demonstrate that we can apply our technique directly to as-grown nanowires used in solar cells,

3,18

spintronic devices,

19,20

electrolytic cells

21

and Majorana devices.

22

We are thus able

to map relevant dopant and impurity levels in the exact nanowires that are used in device applications. APT is based on the evaporation of atoms in the form of ions from a single tip-shaped sample by means of an electric eld. During the analyses ions are projected from the tip apex onto a position-sensitive single ion detector

23

by the electric eld. Based on the mea-

sured positions and the time-of-ight between the tip apex and the detector surface a 3D

2


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reconstruction of the analyzed volume is created.

24

APT is uniquely suited for investigat-

ing the atomic make-up of nano-structures and devices like transistors, or nanowires

711,13

25

nanoparticles

26

as it is unrivaled in its ability to chemically identify a large portion of

the atoms of a volume containing up to a few billion atoms.

However, as APT relies on

(eld-)evaporating atoms from a single tip-shaped object to carry out the tomographic analysis,

24

it is necessary to isolate a single nanowire to allow for a success- and meaningful APT

analysis of a nanowire.

1217

Dierent approaches have been developed to isolate single nanowires for APT including focused ion-beam (FIB) based procedures,

10,11

growing nanowires in isolation

ing up of single nanowires using electron beam induced metal deposition. FIB based procedures can alter the nanowires make-up. in devices are usually grown in nanowire forests

1,3,21,22

27

1217

7,8,13

and pick-

Unfortunately,

Furthermore, as nanowires used

the growth of nanowires in isolation

does not allow to cross-correlate data from APT with device data or use APT to monitor device fabrication. In order to isolate single nanowires from nanowire forests nanowires were welded to a nano-manipulator and subsequently picked up.

1217

However, while it has been postulated

that APT analyses on nanowires grown in isolation can in principle allow for ppm-level characterization of impurities,

28

APT studies on nanowires picked up from forests have not

yet demonstrated sensitivities below the percent level.

1217

Here, we introduce a pick-and-

place approach (gure 1) allowing us to isolate a single nanowire from a dense forest of as-grown nanowires and place it on an etched Si post.

29

We demonstrate that our procedure

creates robust samples suitable for APT from the same nanowires that are used for electrical and optical devices. This allows us to perform measurements at near atomic spatial resolution with parts-per-million sensitivity (gures 2 to 4, supplementary gures 3 to 5) from a wide range of technologically relevant semiconductor nanowires. We developed a four step pick-and-place procedure illustrated in gure 1 to isolate a single nanowire from a forest of nanowires. In the rst step a dummy nanowire is picked up by

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electron beam induced welding

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the dummy nanowire to a nano-manipulator. The dummy

nanowire is taken from a forest of nanowires mounted perpendicular to the manipulator (gure 1a). In the second step the dummy is used to pick up the target nanowire from a forest of the target nanowires such that the two wires form an L-shape (gure 1b). The nanowires are once again welded using electron beam induced metal deposition. In the third step both of the nanowires are welded to the etched Si post (gure 1c) and in the fourth step the dummy nanowire is broken o the target nanowire leaving behind only the target nanowire on the etched Si post (gure 1d).

Please note that the process depends on the

dummy nanowire having a larger tensile strength (allowing us to pull o the target nanowire from the substrate) than shear strength (allowing us to break it o the target nanowire in the nal step), and that a few 100 nm of the target nanowire are lost during the preparation as they are covered by the weld. We typically use 2-3

µm

long InP nanowires with a radius

of 50 - 150 nm as dummy nanowires and adapt the radius of the dummy to the radius of the target nanowire. Our approach has a number of advantages with respect to previous approaches.

714

Just

as previous approaches based on picking up single nanowires using electron beam induced metal deposition,

1214

we do not use a FIB or any coatings making the approach easy to

implement and any chemical alteration of the nanowire unlikely. of our pick-and-place method are:

The additional benets

that the target nanowire is only stressed at the base

during the pick or place procedure while the removal of the nanowire from the substrate by either pulling or bending as done previously is likely to result in signicant stress and may introduce defects; that the target nanowire is welded upside-up onto the Si post (gure 1d, as opposed to upside-down

1214

); that the approach introduced here typically allows for

better alignment of the nanowire with respect to the post, faster isolation of the nanowires, more reproducible welds and less contamination of the target nanowire as discussed in detail in supplementary section S1.

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(a)

(b)

(e)

(c)

(d)

(f)

Figure 1: Procedure for isolating a single nanowire for APT: First, a dummy nanowire (blue) is picked up with a nano-manipulator (green) (a). Second, the dummy is used to pick up the target nanowire (red) (b). Third, both of the nanowires are transferred to an isolated Si post (yellow) (c). Finally, the dummy nanowire is broken o the target nanowire leaving behind only the target nanowire on the etched Si post (d). The procedure can also be used to isolate nanowires grown under and angle (e), as shown here for an InSb nanowire, or branches of nanowire trees (f ), as shown here for a Ge branch. The isolated nanowires are well aligned with the post (inlets) and APT data (e,f ) can readily be acquired.

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The pick-and-place procedure introduced here can be extended straightforwardly from nanowires grown vertically on the substrate (gure 1b) to nanowires grown under an angle (gure 1e) or branches of nanowire trees (gure 1f) by simply mounting the growth substrate under the appropriate angle with respect to the nano-manipulator. We have applied our approach to nanowires composed of the majority of industrially relevant semiconductor materials including Si, Ge, InP, GaP, InAs and InSb (gures 1 to 4, supplementary gures 3 to 5) from dense and sparse forests as well as diameters and lengths varying from 30 to 200 nm and 2 and 16

µm

respectively (supplementary gure 5). Please note that APT typi-

cally does not allow for analyzing tips with an apex radius signicantly larger than 200 nm.

31

Hence our approach is general in the sense that all wires that are thin enough to be measured without an extra sharpening step can also be isolated. We expect that the approach can be readily extended to nanowires grown from other material classes. The welds we create between the target nanowire and the Si post allow for sucient thermal conductance to acquire mass spectra with a very low noise level using laser assisted APT on untapered nanowires (compare gure 2 to gure 7 in reference 28). This demonstrates that our approach in combination with a laser operating at 355 nm can mitigate the challenges arising from the limited thermal conductance of untapered nanowires

28

and

allows for a successful application of laser assisted APT even to untapered nanowires. In addition, the weld and the good alignment of the wire with the Si post enable us to choose analysis parameters that minimize the background noise level in the mass spectrum (supplementary material S2) and eld evaporate nearly the full nanowire - thus collect large data sets (supplementary gure 2) typically containing more than 10

8

ions from single

nanowires. Note that a lower base temperature of the sample, a lower laser power used to assist the evaporation of ions

32

and a higher ion detection rate result in a larger eld on the

tip surface and hence make it more likely that the analysis fails or is interrupted due to fracturing of the nanowire or the weld.

31

The combination of large data sets acquired from single

wires at optimized measurement conditions is what enables us to detect and map impurities

6


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in the nanowires with ppm-sensitivity while maintaining the excellent spatial resolution APT is known for

3335

as discussed below.

The sensitivity to impurities in APT is limited by the background noise acquired during the analyses and statistics.

36

As ions are acquired one at a time during an APT analysis, it is

necessary to collect a sucient amount of ions to allow for reaching a given detection level. In other words, it is necessary to acquire at least a million ions/atoms before a detection limit of one ppm can possibly be reached. Due to the addition of the background noise approximately 5-10 million atoms are usually needed to achieve ppm sensitivity as demonstrated in gures 2 and 3. Figure 2 and supplementary gure 5a show analysis results acquired on nominally pure Ge nanowires grown by the Vapor-Liquid-Solid (VLS) method.

4

We carried out APT analyses

of multiple nanowires from a series of three samples grown with dierent concentrations of residual III-V impurities in the CVD reactor chamber.

The following method was used:

after the growth of InGaP nanowires the CVD reactor was cleaned by supplying Disilane

◦ ◦ at 900 C for 3 hours, then Germane and Disilane at 650 C for 1 hour, and then the rst Ge nanowires sample was grown. Next, the full procedure was repeated to grow the second, third and fourth sample of the series.

From the time of ight spectrum acquired during

the APT analysis shown in gure 2a, we isolate the P peaks as shown in gure 2b. Using the procedure discussed in supplementary S4, we nd a P impurity level of

41.2 ± 1.3

ppm,

7.9±1.3 ppm and 2.0±0.2 ppm in the three growths showing that the impurity concentration decreases as the reactor gets cleaner and that the measured concentration is reproducible. We analyze nanowires atom-by-atom along the growth axis typically starting from the top of the Au droplet. The 3D atom map of the top part of a nanowire from the third sample of the series containing

2.0±0.2 ppm or 1x1017

atoms/cm

3

and a table of impurities detected

in and adjacent to the Au droplet are presented in gure 2c. A number of impurities can be found in the droplet that were most likely present in the growth reactor as a result of previous growths of III-V nanowires. The APT analysis shows that Ga, In and Cu are present in the

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Figure 2:

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(a)

(b)

(c)

(d)

APT of Ge nanowires at ppm-sensitivity:

We evaporate the atoms from the

nanowire one by one and collect a time-of-ight mass spectrum (a).

We can see clear P

signals in the mass spectrum originating from double and single charged P ions as marked by the green and blue arrow in (a) and shown magnied with a ner binning in (b). A 3D volume can be reconstructed from the detected atoms

24

(c) - for clarity only 10 % of the Ge

and Au atoms are shown. This allows us to quantify the level of impurities inside a specic volume as shown for the Au containing regions (c). The 5 nm underneath the Au droplet are contaminated with a few ten atoms of Au and a few P atoms (c,d).

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Au droplet, while P is enriched in a droplet.

∼

2 nm thin layer located

∼

3 nm underneath the Au

We expect that the 5 nm thick Au contaminated region underneath the droplet

is grown during the cool-down procedure and that the P enriched layer is a result of the cooldown.

37,38

Please note however, that only about a hundred-thousand atoms have been

acquired from the Au droplet and only a few ten-thousand from the adjacent regions (table in gure 2c).

Hence, in-spite of a noise free P signal in the adjacent region (gure 2d) a

quantication of the impurity concentration at the ppm-level is not possible in this regions. The data-set of the entire Ge nanowire covers more than 11 atoms (supplementary gure 2a) including

µm

and contains about 1.5e8

254 ± 36 P impurities in addition to those already

accounted for in the table. No Au, Cu, Ga or In is detected in the rest of the nanowire. This indicates that Ga, In and Cu are enriched and largely retained in the Au droplet during the VLS-growth while P is incorporated into the nanowire. The sensitivity to impurities demonstrated in this work is to our knowledge both the highest sensitivity demonstrated on nanowires as well as in APT in general to-date.

28

Note

that we can detect as little as ten impurity atoms inside the Au droplet and as little as a few hundred impurity atoms inside the more than 11

µm

long segment of the Ge nanowire

we analyzed. Impurities in nanowires are usually investigated using EDX or EELS mapping in TEM

21,39,40

reaching sensitivities down to 0.1 %.

5

Our APT analysis hence surpasses this

limit by nearly 3 orders of magnitude and establishes that APT is capable of detecting and mapping impurities in nanowires relevant for device applications at the ppm-level. APT data are inherently 3D and as discussed above we can acquire more than onehundred million atoms from single nanowires but only 5-10 million atoms are needed to achieve ppm sensitivity. Hence, we can subdivide the 3D volume reconstructed from the APT data to create proles of the impurity concentrations. A prole of the P concentration in the Ge nanowire shown in gure 2c and supplementary gure 2a is shown in gure 3a. Each of the subvolumes used for the proling contains between 6-7 million atoms and corresponds to a 500 nm long nanowire segment. As shown in gure 3a the P signal in nearly all subvolumes

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is still statistically signicant and the nanowire contains an approximately constant level of P at 2.0 ppm or 1x10

17

3 atoms/cm . Similar proles along the depth and the radial axis of

Al impurities in a GaP nanowire, P impurities in an InAs nanowire and a Si doping prole in an InP nanowire are shown in supplementary gures 3-5 demonstrating that the method is able to map doping and impurity levels at the ppm-level in a wide range of semiconductor nanowires. The limitations resulting from the counting statistics

36

discussed above are what limits

the spatial resolution that can be achieved when mapping impurities.

This is highlighted

in gure 3. Note, that the proles in gure 3a and supplementary gures 3-5 are generated using millions of atoms per data point which are collected from segments of nanowires that are several 100 nm long. In gure 3b we show an APT analysis of a 1.4 nm wide (at full width half maximum, 2.9 nm at full width tenth maximum) InGaSb tunnel-barrier in an InSb nanowire.

42

As the dimensions of the barrier are only a few nanometer, using segments

of a few hundred nanometer to create the depth prole is ill-advised. Lowering the bin widths results in a smaller amount of atoms used to calculate the concentrations of each data point. In order to prole the barriers we chose bins of 0.1 nm width and each prole point is generated by only approximately 1000 atoms. As a result the error bars resulting from the counting statistics

36

are approximately 0.1 % or 1000 ppm. In addition, as demonstrated

by the carbon signal in gure 3 single noise counts result in an apparent concentration of about one in a thousand which is again 0.1 % in the respective bin and the lack of statistics makes a (local) background correction impossible. Note, that in principle this limitation can be overcome by measuring a large number of these tunnel-barriers and averaging over the results. Our approach for the preparation of single nanowires for APT hence enables us to acquire data that are mainly limited by fundamental counting statistics and the noise induced in the detector. Hence, as the eciency of the single-ion detection systems (in our tool 36 %) improve and methods for decreasing the background noise in the detector signal are devel-

10


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(a)

(b) Figure 3: Concentration proles along the growth axis showing P impurities in a Ge nanowire (a) and a InGaSb tunnel-barrier with a width of 1.4 nm wide (at full width half maximum, 2.9 nm at full width tenth maximum) in an InSb nanowire (b) highlighting the impact of counting statistics and noise. The top prole (a) is created by binning 500 nm long segments containing 6-7 million atoms allowing us to prole a 2.0 ppm signal and revealing that the Au concentration in the nanowire is below the ppm-level - error bars are on the order of 0.5 ppm. No detectable amount of Au is present in the nanowire. The bottom prole (b) is created by binning 0.1 nm segments containing approximately 1000 atoms degrading the detection limit as error bars are now on the 0.1 % or 1000 ppm. This is demonstrated both by the Ga and the C prole. There is no detectable amount of C present in the nanowire but single noise counts give a local concentration measure of about 0.1 %/1000 ppm. The As content is the result of the growth procedure.

41,42

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oped

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our preparation method will allow us to map impurities in nanowires ever closer to

the ultimate statistical limit. It is necessary to nd a tradeo between sensitivity and spatial resolution when applying APT analyses to nanowires. Mapping impurities at ppm-sensitivity can currently only be carried out on subvolumes that contain more than 5 million atoms and hence volumes of several hundred-thousand cubic nanometer. However, the inherent spatial resolution is in the sub-nm range as demonstrated in gure 4a on a Ge nanowire from the series discussed in gure 2. In the reconstructed atom-by-atom data-set both a (111)-plane-set (middle) and a (110)-plane-set (right) are clearly visible. The planes are visualized using spatial distribution maps which are generated by summing over the relative distance between each atom and its neighbors.

34

The quality of the imaged planes indicates a depth resolution of 0.15 nm

and a lateral resolution of 0.3 nm.

33

The spatial resolution achieved in pure Ge extends to

the analysis of core-shell nanowires as shown in a comparison between an APT and crosssectional EDX mapping in a TEM of Ge-core, Si-shell nanowire shown in gure 4b.

The

steepness of the prole achieved in both methods is comparable while the signal-to-noise ratio in APT is signicantly better. The data quality in TEM is however limited by both the FIB based preparation necessary to extract the nanowire cross-section and the sensitivity of the EDX detection system. Note, that the sub-nm spatial resolution demonstrated here is also benecial for the mapping of impurities at any level as it allows to create subvolumes along certain features of the sample (e.g. a core-shell interface) with high accuracy. In conclusion, we have shown that we have developed a pick-and-place procedure to isolate single nanowires from forests of nanowires allowing us to perform APT on a wide range of nanowires that are used in devices. Nanowires shown in this paper include amongst others InSb nanowires that are used for Majorana physics, used in solar cells,

3,40

22

InP nanowires that can be

GaP nanowires that are used for water reduction

shell nanowires that are used in spintronics.

19

21

and Ge-core, Si-

Our data sets allow us to detect and map

the presence of impurities at the ppm concentration-level, an improvement of about three-

12


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(a)

(b) Figure 4: Spatial resolution of APT on nanowires evaluated by imaging crystal features (a) and by comparison with chemical mapping in TEM based EDX (b). In the reconstructed atom-by-atom data of the Ge nanowire (a, left) a (111)-plane-set (a, middle) and a (110)plane-set (a, right) is resolved.

34

This quality of the imaged planes indicate a depth resolution

of 0.15 nm and a lateral resolution of 0.3 nm.

33

The comparison of the concentration prole

over a Si/Ge interface in a Ge-core, Si-shell nanowire (b) taking from APT and cross-sectional EDX mapping in TEM demonstrate that the lateral resolution achieved here is comparable to EDX as the abruptness of the interface is comparable in both data sets.

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orders of magnitude over the to-date's standard TEM-based approaches. Note that we have demonstrated ppm-level sensitivity by: multiple measurements of each growth from a series of Ge nanowires containing decreasing amounts of P at the ppm-level (gure 2), proling of impurity at the ppm-level in dierent nanowires (gure 2 and supplementary gure 3-5), and showing signals that drop below the ppm-level for P in Ge, Au in Ge and Si in InP (gures 2 and 3 and supplementary gure 5). Hence showing that our analyses are reproducible, signals are consistent throughout a data set and the absence of a signal results in a quantication error below the ppm-level.

Furthermore, we are able to image interfaces in the nanowire

with a quality comparable to TEM based analyses.

Our approach allows us to supervise

and hence optimize the growth of nanowire structures as we have already demonstrated in another paper by growing record hole-mobility Ge-Si core-shell nanowires.

44

Given the wide

range of applications for nanowires from solar cells to quantum computing discussed in the introduction and the generality of our method, we expect that the procedures developed here will nd wide application as APT analyses of nanowires will make a signicant contribution to supervising, understanding and optimizing the growth of nanowires and hence to improving the performance of nanowire-based devices in the near future.

Acknowledgement The authors thank Vavara Efremova, Anna Ceguerra, Simon Ringer and Peter Felfer from the University of Sydney for the development of the AtomBlend plug-in.

This work was

partly nanced by The Netherlands Organization for Scientic Research (NWO)

Methods APT was carried out using a LEAP 4000X-HR from Cameca. The system is equipped with a laser generating picosecond pulses at a wavelength of 355 nm. For the analysis, all samples were cooled down to a temperature of 20 K unless mentioned otherwise. The experimental

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data are collected at a laser pulse rate of 65-200 kHz. Ge, InSb and Ge-core Si-shell nanowires were analyzed at laser powers between 2 and 6 pJ; GaP nanowires at laser powers between 1 and 3 pJ; InAs and InP nanowires at laser powers between 0.4 and 0.8 pJ. APT data have been reconstructed using IVAS 3.6.8 and visualized using AtomBlend in Blender 2.7.6. All Nanowires have been isolated in a FEI Nova Nanolab 600i. electron induced metal deposition of Pt or Co.

Welds are created by

30

InP and InSb nanowires were grown using metal-organic vapor phase epitaxy (MOVPE) horizontal laminar ow reactor from Aixtron. The self-catalyzed InP nanowires are grown on an InP substrate

40

using a growth mask with 50 nm thick SiN x with 180 nm wide openings

at a distance of 513 nm, at a temperature of 730

◦

C and a V/III ratio of 83.

prole is created by adding Ditertiarybutylsilane to the growth chamber.

A doping

InSb nanowires

are grown from an Au catalyst particle on an InP substrate using a procedure similar to the one described in reference 41. All other nanowires were grown in a MOVPE system with the close-coupled showerhead technique provided by Aixtron. GaP nanowires are grown from an Au catalyst particle on an GaP substrate following the procedure discussed in reference 39 using a growth time of 180 min. InAs nanowires were grown from an Au catalyst particle on an InP substrate for 18 min at 540

◦

C using trimethylindium and arsine as precursors. Ge and Ge-core Si-shell

nanowires were grown from an Au catalyst particle on a Ge substrate using GeH 4 and Si2 H6 as precursors and HCl as an additive.

44

The following method was used to purge the reactor

in between the growth of dierent series of Ge nanowires: after the growth of a series of

◦ nanowires the reactor was cleaned by supplying Disilane at 900 C for 3 hours and then ◦ Germane and Disilane at 650 C for 1 hour in order to cover the reactor walls and suppress the supply of impurities from the walls to the vacuum background. For TEM the nanowires were transferred mechanically to holey carbon lms and analyzed by a JEM-ARM 200F high resolution TEM operating at 200 keV. The microscope is equipped with an EDX system with a

1002

mm Si drift detector.

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Supporting Information Available Supplementary material Additional information on the isolation procedure, the optimization of the APT analysis parameters for minimum noise level and the background correction employed for the data treatment are given.

APT analyses o large sections of nanowires with thicknesses from

30 to 200 nm are demonstrated and impurity proles in GaP, InAs and InP nanowires are shown.

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Atom-by-Atom Analysis of Semiconductor Nanowires with Parts Per

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