1. d ~ r Results Characterization of GaAsP NWs grown on Si substrates
S. Ali1, S. Rybchenko1, H. Liu2, Y. Zhang2, S.K Haywood1
1. School of Engineering, University of Hull, Cottingham Road Hull, HU6 7RX
2. Department of Electronic and Electrical Engineering, University College London, Torrington Place London WC1E 7JE
School of Engineering
Sample preparation
Motivation
By integrating direct band gap III−V materials onto the mature and cost-effective Si platform, high absorption coefficients, high carrier mobilities, and large solar spectrum coverage could be achieved, creating novel optoelectronic devices for Si photonics.
This integration remains challenging due to the large lattice mismatch and the difference in thermal expansion coefficient.
In planar epitaxy a lattice mismatch of less than 1% leads to a degradation of layer quality due to the formation of misfit dislocations
In recent years, one-dimensional semiconductor III−V nanowires (NWs) have gained significant attention for integrating III−V materials and devices on a Si platform because of their unique structural, optical, and electronic properties.
The strain formed between III−V NWs and Si substrates can be effectively relieved over a thickness of a few monolayers due to a small contact area and large surface-to volume ratio.
It has been predicted that a two-junction tandem solar cell, consisting of a 1.7 eV NW (GaAsP) junction and a 1.1 eV Si junction, has a theoretical efficiency of 33.8% at 1 sun AM1.5G and 42.3% under 500 suns AM1.5D concentration.1,2
Two pieces of sample glued face to face, then four dummy Si sample glued on its sides (two in each side). After the glue is set, 2.5 x 2.2 x 0.5 cm square piece, cut by diamond saw
One side of the sample is griound using a manual grinder and then polished using an electric polisher
Second side of the sample is ground manually until its thickness is reduced to nearly 100um and then polished using an electric polisher
Then sample is ion milled using a PIPs machine at 5keV, 3rpm, single side beam modulation. Next the sample is characterised by TEM
SEM
Results
EDX
Raman
EDX analysis is performed on GaAs(1-x)Px NWs
No trend in material composition along the length of NW within EDX accuracy i.e.
x = ± 0.02
where
Also ratio of III/V is close to one
NWs can be revealed by strong LO/SO peak as comparted to TO phonon peak.
PL intensity map appears to be complementary to the Raman intensity distribution. The reason for this is not very clear at the moment.
Fig.1 SEM image of GaAs(1-x)Px NW/Si at 10k, 30o tilt
Area of measurement = um2
NW density: NWs per cm2
TEM---- Detached NWs
Most of the faults are at the tail and middle part of the NW
However the head is defect free
GaAs(1-x)Px NWs sample, ultrasonically bathed in acetone. Drop of acetone solution contacting NWs is placed on lacy carbon TEM grid.
Long & thin NWs
Length=7-8um
Diameter = 50-60nm
Short & fat NWs
Length=3-4 um
Diameter = nm
Tail
head
Extended areas in NW which are defect free
TEM---- Cross Section of NWs
Strained
Relaxed
d > r’
d ~ r
Cross section samples of NWs are prepared using method described in sample preparation NW
Si Substrate
Residual material
NW-substrate interface is strained (no dislocations).
Residual material-substrate interface is relaxed (having dislocations)
Strain patterns under each NW
Results Summary
Two type of NWs in our GaAs(1-x)Px sample, ‘short fat’ and ‘long thin’
NWs material composition remained the same along its entire length in both types.
Large density of stacking faults and twins are observed within the NW; majority of the defects are at the tail of the NW
In a cross-sectional study, NWs are standing strained and the area of the strain is similar to the diameter of the NW, so there must be no dislocations at NW/Si interface which is proved in our high magnification TEM results.
However, residual material deposited at the time of growth has strain patterns less than the size of their diameter, hence there must be dislocations in it. A further study is on-going to investigate these dislocations.
References
(1) LaPierre, R. R. J. Appl. Phys. 2011, 110,
(2) Geisz, J. F.; Friedman, D. J. Semicond. Sci. Technol. 2002, 17, 769
At low magnification, strain patterns at both sides of the NW/Si interface
At high magnification, NW/Si interface has no dislocations