Prior to the articles that follow, Ga2O3 had received little attention as a functional
layer of any kind in silicon based electronics. Gallium oxide had been shown to develop a significant charge density at the interface with strained silicon (a silicon/germanium alloy) of 6×1012 cm−2 [62], however it wasn’t until the independent observations of capacitance–voltage measurements by Altuntaset al. and this author that a high nega- tive charge density was reported on metal-insulator-semiconductor (MIS) test structures fabricated with ALD deposited Ga2O3 on silicon substrates [50],[63],[64]. A reduction of
the density of interface defects was also first reported by the author in [50],[64], together providing further insight into the surface passivation results reported in [65],[66].
In recent years the electrical properties of Ga2O3 – it’s wide bandgap, UV transparency,
high electrical field strength, oxygen vacancy conductivity, and ability to be doped for n- type conductivity – have inspired investigations into the use of for the fabrication of thin film transistors [60], high temperature gas sensors [67],[68], and solar-blind photodetector applications [69],[70]. Investigations into the use of Ga2O3in PV applications on cuprous
oxide [52],[71], CIGS [72], and dye sensitised absorber materials [73] have also been performed. The articles that follow are the first investigations carried out in the area of c-Si solar cells.
One of the first PV applications of Ga2O3was that of Chandiranet al., who used an ALD
deposited Ga2O3 tunnel layer as a means of blocking the back reaction of photo-excited
electrons between a porous TiO2 scaffold and the absorbing dye [73]. The application of
Ga2O3 resulted in a remarkable increase in device open circuit voltage from 690 mV to
1.1 V. The authors later demonstrated that it is the favourable band structure of Ga2O3
and conduction band offset with the TiO2 layer that were of particular significance in
this context [74].
The application of Ga2O3 to cuprous oxide (Cu2O) followed; the interest stemming
from a favourable band alignment of electron collection. In the work of Minami et al., a Ga2O3 electron collecting layer up to 80 – 90 nm thick was shown to provide optimal
performance (η∼5.4%) indicative of bulk conduction and a favourable conduction band alignment [75]. Lee et al. would later measure a Voc of 1.2 V on a Ga2O3 passivated
XPS measurements, the cell suffered from very high series resistance losses measuring an efficiency of <4% and a fill factor of only 44.7% [52].
Most recently, Ga2O3 was applied to CIGS (Cu(In,Ga)Se2) devices as a cadmium-free
alternative to the state-of-the-art electron collector, CdS. Koida et al. demonstrate comparable open circuit voltages using up to 80 nm of Ga2O3 in such an arrangement,
though reduced fill factors, a result of larger barrier heights at the CIGS/Ga2O3interface
3.4
Foreword
The journal papers that follow represent the first attempts to investigate the application of Ga2O3 to c-Si PV applications, with a focus on using Ga2O3 to passivate c-Si surface
defects. The first paper, a letter published in Applied Physics Letters, is the first to demonstrate surface passivation by Ga2O3. The investigation used trimethylgallium
(TMG; the gallium analogue of the commonly used Al2O3 precursor, TMA) and ozone
(O3) as reactants in a thermal ALD process. Thermal activation of the passivation, and
subsequent de-activation, are studied.
The subsequent papers detail investigations into surface passivation by plasma ALD, owing to the higher deposition rate and improved passivation properties when replacing O3 with O2 plasma as the oxygen source. From high frequency capacitance–voltage
measurements, the surface passivation is shown to be a result of a reduction in the density of surface defects (midgap defect density<1011 eV−1.cm−2), and the formation of a high density of negative charge in the film (>1012cm−2) after annealing, similar to state-of-the-art Al2O3. This is shown to result in an equivalence of surface recombination
parameters for Ga2O3passivated silicon surface, compared to Al2O3, on both undiffused
and boron diffused p+ surfaces.
In the third paper presented below the Ga2O3film is shown to act as a suitable Ga doping
source in a laser doping processes. Proof-of-concept p-type cells with Ga-doped partial rear contacts are fabricated with rear Ga2O3 surface passivation. The resulting device
efficiency of 19.2% was hampered largely by a low fill factor 74.5% which is attributed to unoptimised laser doping conditions. The open circuit voltage of the device (658 mV) is commensurate with Al laser-doped devices from Al2O3 sources reported in the
The demonstration of surface passivation is also extended to random pyramid textured surfaces and PECVD silicon nitride capping for applications to the front side of n-type solar cells, for example. Capping with PECVD SiNx is shown to both improve surface
passivation and thermal stability, with initial investigations indicating that the Ga2O3
/ SiNx stack is firing stable. From x-ray diffraction data the SiNx capping layer is
3.5
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