before and after APT and using known values such as plane spacing. There are furthermore plans to combine APT with electron microscopy in the one instrument so that the tip shape may be imaged during the APT process.946 Another potential artefact is the migration of surface atoms before ionisation. Migration may be particularly problematic for dilute species with higher ionization potential than that of the host matrix. Finally, the ionisation of multiple ions or clusters can generate ambiguity in identifying the evaporated species. A comprehensive book on the subject of APT has been written by Gault et al.948 The current study employed a laser-assisted local- electrode atom probe system (LEAP) (Cameca 3000X Si λ= 532 nm, repetition rate 200 kHz, pulse length 10 ps) equipped with a position-sensitive, wide field-of-view detector and a micro-electrode for ion extraction. Characterization was performed at 20 K using laser energies ranging between 10 and 100 pJ.
3.4 Photoluminescence spectroscopy (PL)
Photoluminescence spectroscopy (PL) describes the measurement and analysis of optical emission generated by optical excitation. In the case of semiconductor materials, photons with an energy greater than that of the bandgap are used to generate electron hole pairs. Emission by subsequent radiative recombination reveals information relating to the electronic structure of the material. Where a microscopic feature or material is of interest, PL may be performed with the aid of an optical microscope in what is known as micro-photoluminescence spectroscopy (-PL). A full review of the topic of PL has been written by Gilliland.949
The custom μ-PL setup used in the current work was built in-house and is illustrated in Figure 3-6. Optical excitation was provided by a pulsed diode pumped solid-state (DPSS) laser (femtoTRAIN IC-Yb-2000, λ = 1044 nm, repetition rate 20.8 MHz, pulse length 400 fs, 3 W maximum output), which was frequency doubled to 522 nm. The excitation intensity was controlled by neutral density filters. Individual nanowires were imaged and excited using an optical microscope (Nikon Eclipse L150) equipped with a motorised stage and a high magnification lens (Nikon LU Plan 100x/0.9NA). Photoluminescence was collected through the same objective lens before being spectrally filtered (550 nm longpass) to remove the laser wavelength. The spectrometer (Acton, SpectraPro 2750) contained two selectable gratings (150 lines /mm or
1200 lines /mm) and spectral data was recorded using a Peltier-cooled CCD camera (Princeton Instruments, PIXIS). For low temperature measurements a continuous flow He-cooled cryostat (Janis research) was mounted on the microscope stage. The cryostat featured a 0.5 mm thick chemically polished 0° sapphire window, which was used in conjunction with an aberration-corrected 60x/0.70NA lens (Nikon CFI Plan Fluor). 3.4.1 Time-resolved photoluminescence spectroscopy (TRPL)
Time-resolved photoluminescence spectroscopy (TRPL) methods measure PL emission as a function of time. Transients are usually generated by pulsed excitation and provide an insight into the dynamical behaviour of semiconductor materials. The current study measured the carrier lifetime of individual nanowires using a form of TRPL known as time-correlated single photon counting (TCSPC). TCSPC involves exciting the sample with a pulsed source such that there is less than a few percent chance of detecting
`
Legend
PBS Partial Beam splitter (~4%) LP Linear polariser
Xtal LBO non-linear doubling crystal GT Glan-Thompson polariser
F1 522nm bandpass filter DCM Dichroic Mirror
F2 550nm longpass filter Flip mounts
Figure 3-6 | Schematic illustration of the layout used for optical spectroscopy. Adapted with permission from Saxena.7
Section 3.4 - Photoluminescence spectroscopy (PL) emission for any given cycle. Repeated measurements of the delay between excitation and emission are then used to generate a frequency histogram which corresponds to the recombination dynamics of the sample. The TCSPC setup is shown in Figure 3-6. A partial beam splitter (PBS) is used to generate a reference beam which is directed to a photodiode and provide the start signal. Emission of a selected wavelength is directed to a single photon avalanche diode (SPAD) to generate the stop signal. Signal processing was provided by a Picoharp 300 with a response time of approximately 30 ps. The maximum window for TRPL was limited by the laser repetition rate to approximately 50 ns.
3.4.2 Sample preparation
Samples for PL measurement were firstly transferred onto alternate substrates by mechanical dispersion. This was achieved by gently rubbing the face of the nanowire growth substrate across that of the alternate substrate which was typically Si. Where guided modes and lasing behaviour were of interest, ITO (indium tin oxide, ∼ 200 nm thickness) coated SiO2 substrates were used in place of Si to minimise absorption and leakage. These substrates were firstly patterned with Au markers such that the same nanowires could be repeatedly identified for PL and later SEM analysis. The markers were defined using a standard photolithography process which has been described in detail elsewhere.7
4
Zinc as a Dopant:
p-type GaAs nanowires
4.1 Introduction
Realizing controlled impurity doping remains a key obstacle to the continued development of semiconductor nanostructures such as nanowires.211, 213, 226, 950 Particular challenges include incorporation207, 951 activation214, 952 and metrology209, 953, 954. Reductions in contact size further demand increased dopant concentrations and improved statistical control over doping.209, 955 These issues are compounded by the unique geometries and nanoscale volumes which preclude or complicate many of the standard quantitative tools of semiconductor engineering such as secondary ion mass spectrometry (SIMS) and Hall measurements.209, 232, 233, 495 In the context of the aforementioned challenges, there exists a rich body of work reporting on the impurity doping of semiconductor nanowires.950 Doping has been found to have a variety of potential effects on nanowire growth including altering the growth rate,220, 244, 246, 449, 498, 956
the stacking fault density and crystal phase,35, 244, 247, 562 changing the growth direction through kinking and interrupting the VLS process itself.237, 445 The potential for significant dopant inhomogeneity has further been identified with incorporation generally being found to be greater for radial relative to axial growth.218, 220, 412, 455, 464, 950 Despite these issues, a wide variety of technically relevant doped semiconductor nanowire devices have now been demonstrated including transistors,99, 377, 470, 499 electrically pumped lasers,117, 957 light-emitting diodes (LEDs)110, 499 and photovoltaic devices.85, 86, 125
This chapter investigates the Zn doping of Au-seeded GaAs nanowires. Working with the precursor diethylzinc (DEZn), various morphological effects of in situ Zn doping are investigated and suitable growth conditions for Zn-doping are identified. The incorporation of Zn and distribution thereof is explored using both EDXS and APT, with APT in particular enabling the quantification of dopant concentrations. The
Section 4.2 - Methods