Crystallographic and electrical analysis of radial-grown
npn-nanowire heterojunction bipolar transistor
Student:
Scientist:
Mark Galacki
Claudia Speich
Introduction
The topic of this bachelor thesis comprises the crystallographic and electrical analysis of
GaAs/InGaP nanowires as a npn-heterojunction bipolar transistor (HBT) and the influence of the
different doping types in a InGaP interface layer between the base and collector region on the
electrical properties of the electrical device. The crystallographic characterization is achieved by
ascertaining the lattice strain in the nanowires through x-ray diffractometry. The electrical
examination includes the measurement of current-voltage characteristic of the heterojunction
bipolar transistor. On top of that a test series with etching solutions for selective InGaP-etching
was conducted.
Experimental Setup
The nanowires are grown trough vapour-liquid-solid (VLS) and vapor-solid (VS) growth
mechanism in the metalorganic vapour phase epitaxy (MOVPE). A colloidal solution with
100 nm gold particles is deposited by spin-coating on an n-doped GaAs substrate with the
crystallographic orientation (1 1 1)B. The cross-sectional width of a nanowire is dependent on the
diameter of the colloidal gold particles (size deviation ± 8 %). The VLS-growth process of the
GaAs occurs at a temperature of 450°C. At a temperature of 650°C layer-growth of the InGaP
through VS-mechanism is realised. In various technological process steps each layer of the HBT
is uncovered by the use of wet chemical etching. The processed nanowires are transferred to a
silicon (Si) wafer with a 5 µm thick layer of silicon dioxide (SiO2) and prestructured golden (Au)
contact surfaces.
Throughout this study two different layer configurations were analyzed. Electrical researches
were done on a layer series (n-GaAs/p-GaAs/n-InGaP) by the working group HLT at the
University of Duisburg-Essen and layer configuration (n-GaAs/x-InGaP/p-GaAs/n-InGaP) with
an additional InGaP interface layer with different doping types used as an etch-stop layer.
For the crystallographic analysis of nanowire heterostructures diffraction images recorded by the
x-ray diffractometry were used. The electrical analysis of the HBT was conducted by measuring
the current-voltage characteristics of the two pn junctions by on-wafer DC measurements.
Results
The influence of the lattice strain in the nanowires was calculated through the angular distance of
secondary peaks in relation the main peak. The main peak corresponds to the substrate peak
(GaAs) and has the lattice strain 0 (no lattice strain). Individual peaks in the diffraction image are
highlighted by lines. The evaluation of the diffraction image depict a strong correlation between
the peak position, which is dependent on the material composition, in the spectrum and the lattice
strain induced by another material composition (in this case lattice-matched InGaP growth on
GaAs). The individual peaks in the spectra of the x-ray diffractometry measurements can be
assigned to the different material systems within the nanowire. On the righthand side of the main
peak a side peak (2θ=27,30˚) is observable. This peak is found in the diffraction image of all
investigated samples and stands for the n-doped GaAs layer. Furthermore in all diffraction images
of samples containing an etch-stop layer two similar side peaks can be found on each side of the
main peak. Those side peaks are assigned to the InGaP material system (see fig. 1). The right
flank belongs to the InGaP intermediate layer (2θ=27,20˚), whereas the peak on the left side
depends on the crystal structure of outer n-InGaP layer (2θ=27,51˚). The left flank of the main
peak can be assigned to the p-GaAs layer.
b)
a)
p-GaAs
InGaP
n-GaAs
InGaP
p-GaAs
InGaP
n-GaAs
InGaP
fig. 1:Diffraction images of a) a sample with n-GaAs/p-GaAs/n-InGaP b) a sample with a n-doped InGaP interface etch-stop layer.
The intensity and position of the left "shoulder" is primarily dependent on the type of doping used
for the interface layer. The n-doped InGaP peaks show higher angular width than intrinsic InGaP.
Due to a greater dopant quantity of the n-doped interface layer a higher lattice strain is expected.
The analysis shows that the cross-section of the lattice-matched InGaP and GaAs layers as well
influence the lattice strain; thicker layers resulting in higher lattice strain. Those correlations
between material composition and material configuration were gradually recorded and replicated
by measuring just the nanowire-core (n-InGaP) and repeating those measurements while adding
one layer after another resulting in peaks dependent on the material configuration.
In order to determine the electrical properties of the nanowire semiconductors current-voltage
characteristics of the two pn junctions, base-collector and base-emitter junction,were recorded.
The input voltage ranged from -2V to +2V graduating in 0,1V steps and the corresponding base
current was measured. All measured samples generated current-voltage characteristics indicating
a functioning pn-junction. All HBTs ,with and without the etch stop layer, produced base currents
having their maximum in the range of milliamperes. Due to problems in the processing stage and
while applying electrical contacts EDX measurements and current-voltage characteristics proved
that many nanowires weren’t connected property resulting in an open circuit.
During this bachelor thesis various etching solutions for InGaP etching were tested in order to
minimize the material residua caused by the HCl etching solution at the crystallographic facets of
the nanowire. The influence of the etching systems will be explained on the basis of evaluating
scanning electron microscope (SEM) images. Oxidative hydrochloric and acetic acid etching
solution (HCl:CH3COOH:H2O2) isn’t completely selective to GaAs and was used in order to
remove the material residua. The etching solution with 1:10:1 proportion has a selectivity of
approximately 4 and etched the nanowire surface evenly. Unfortunately, the SEM images don’t
reveal the difference in the materials and the etching rate couldn’t be determined.
Conclusion
The crystallographic analysis shows that certain peaks of the diffraction image can be assigned to
the specific layers in the nanowire. In addition to that, the cross-section width and the doping type
influence the lattice strain and thus the angular peak width and peak position of the material
configuration. Hence, the diffraction images of nanowires offer the possibility to make rough
conclusions on crystal properties of the materials used in the nanowire.
The quantity of the electrical measurements is insufficient to make a specific statement about the
impact of the InGap etch stop layer on the maximum electric current produced in the HBT.
Nevertheless the electrical measurements of the BC- and BE-junctions were compared with
Shockley-equation of an ideal Si diode and BC-junctions of the nanowires with the InGaP
interface layer showed a worse emission coefficient than those without the layer. This leads to the
assumption that the InGaP interface layer has negative effects on the current-voltage
characteristics of the BC-junction.