Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

GaAsN as a photovoltaic material – photoelectrical
characterization
Paulina Kamyczek 1) , Ewa Placzek-Popko 1) , Piotr Biegański 1) , Eunika Zielony 1) ,
Beata Sciana 2) , Marek TłaczaŁa 2)
1)
Institute of Physics, Wroclaw University of Technology
2)
Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology
GaAsN and InGaAsN materials have attracted considerable attention
due to their unique physical properties and wide range of
their possible application in optoelectronics, especially in infrared
laser diodes for 1.3 and 1.55 mm [1, 2] and high efficiency
multi-junction (MJ) solar sells [3, 4], where these low-band-gap
materials can, in principle, be used to efficiently collect the lowphoton-energy
portion of the solar spectrum [4].
In this paper the results of studies on the layers of GaAs 1-x
N x
grown on (100)-oriented Si-doped n-type GaAs substrates by atmospheric
pressure metal organic vapour phase epitaxy APMO-
VPE are presented. In the first step the layers of GaAs 1-x
N x
were
characterized with the use of optical methods, then Schottky diodes
were realized and the rectifying properties of the diodes were
studied with electrical methods. The diodes exhibit light-energy
conversion effect. Basic parameters of the solar cells ( short-circuit
current I sc
, open circuit-voltage, V oc
, and fill factor, FF) were
determined. Obtained results confirm that the diodes are promising
as efficient solar cells.
Samples
The investigated structures were grown on (100)-oriented Sidoped
n-type GaAs substrates by APMOVPE with AIX200 R&D
AIXTRON horizontal reactor. Trimethylgallium (TMGa), tertiarybutylhydrazine
(TBHy) and arsine (AsH3) were used as the growth
precursors. High purity hydrogen with the total flow rate of
9.6 l/min was employed as a carrier gas. The hydrogen flow rate
through the saturator was changed during runs with TBHy - VH2/
TBHy = 1500...2500 ml/min. Stable parameters were as follows:
the growth temperature was Tg = 566°C , the arsine flow rate
VAsH3 = 50 ml/min (for GaAsN) and 300 ml/min (for GaAs), the
organic source temperatures: TTMGa = −10°C, TTBHy = 30°C.
The thickness of Si-doped n-type GaAs substrates was set to
350 μm. Subsequently GaAs buffer layer of the thickness of 450
nm was grown. The layer of 200...300 nm thick GaAs 1−x
N x
was
grown on top of the buffer layer. Gold Schottky contacts of 0.5
mm 2 area were prepared by electrolitography technique on the
front side of the GaAs 1−x
N x
layer. An AuGe served as the ohmic
contact to the n-type GaAs substrates (cf. Fig. 1).
In this paper two kinds of samples labeled as N42N, N48N with
various nitrogen content were investigated. Nitrogen content was
determined from the spectral characteristics of transmittance (T) ,
reflectance (R) and photocurrent (PC).
Experimental
The optical properties were analysed using transmittance, reflectance
and photocurrent spectral room-temperature (RT) measurements.
The transmitted and reflected light was dispersed by
BENTHAM spectrometer and detected by the Ge and Si detectors
with a lock-in amplifier. The same system was used to perform
photocurrent measurements. The latter were realized on Schottky
Au–GaAs 1−x
N x
/GaAs diodes. The solar cell figures of merit (Isc,
Voc and FF) were determined from current-voltage characteristics
carried out in darkness and after illumination with halogen lamp.
The dark and illuminated current-voltage (I-V) characteristics
were measured by using Keithley 2601 current source meter.
From transmittance or reflectance spectra the excitonic band
gap can be obtained. The first derivative of the absorption coefficient
has a maximum in the vicinity of the band gap [5] whereas
reflectance spectrum exhibits a “dip” at the same wavelength. The
value of the band gap (GaAs 1-x
N x
layers) can be also determined
from the photocurrent spectrum more specifically from the midpoint
of the PC drop. Once the energy of the excitonic band gap
is determined, the GaAs 1-x
N x
nitrogen content x can be extracted
from the equation:
2/3
∆ E = 3.91x
(1)
where ΔE g
= E g
(GaAs) – E g
(GaAs 1x
N x
) [5, 6] (assuming RT E g
(GaAs) = 1.42 eV).
From I-V characteristics basic parameters characterizing solar
cell can determined: open circuit voltage V OC
and short circuit current
I SC
and a point of maximum power P max
. Using this parameters
a fill factor FF can be calculated:
(2)
Results and discussion
g
Optical properties of the studied heterostructures were verified by
the room temperature (RT) transmittance (T), reflectance (R) and
photocurrent (PC) spectral measurements. A comparison of the
reflectance spectra with the derivative of transmission for N42N,
N48 are given in Fig. 2a and 2b. The arrows correspond to the excitonic
transition of GaAs 1-x
N x
and the dotted lines indicate GaAs
band gap shifted with respect to the expected value of 1.42 eV
due to Urbach tail.
Figure 3 shows PC spectra of the structures with different nitrogen
content in the GaAs 1−x
N x
epilayers (N42N and N48N). The
photocurrent generated in GaAs 1−x
N x
can be seen above the absorption
edge of the GaAs. In Table 1 the nitrogen content x obtained
for the studied samples (N42N, N48N) from transmittance
and reflectance as well as from PC measurements are collected.
Tabl. 1. Nitrogen content obtained for the studied samples
Sample x (%) T x(%)PC
N42 1.08 1.2
N48 1.88 1.9
Fig. 1. A diagram of a GaAs 1−x
N x
/GaAs heterostructure
The dark and illuminated I-V curves of the two structures are
shown in Fig. 4, while the basic solar cell parameters of the diodes
obtained from the curves are reported in Table 2.
Elektronika 6/2012 109