Photovoltaic Properties of p-Doped GaAs Nanowire Arrays Grown on n-Type GaAs(111)B Substrate
© to the authors 2009
Received: 5 October 2009
Accepted: 30 October 2009
Published: 14 November 2009
We report on the molecular beam epitaxy growth of Au-assisted GaAs p-type-doped NW arrays on the n-type GaAs(111)B substrate and their photovoltaic properties. The samples are grown at different substrate temperature within the range from 520 to 580 °C. It is shown that the dependence of conversion efficiency on the substrate temperature has a maximum at the substrate temperature of 550 °C. For the best sample, the conversion efficiency of 1.65% and the fill factor of 25% are obtained.
Aligned NW arrays are very promising building blocks for various nanoelectronic devices, such as nanolasers [1, 2], field-effect transistors , light-emitting diodes [4, 5] and field emitters [6, 7]. Compared to polycrystalline films, vertically oriented NW arrays are particularly advantageous for photovoltaic (PV) application, because the oriented geometry provides direct conduction paths for photo-generated carriers to transport from the junction to the external electrode, thereby resulting in a high carrier collection efficiency [8, 9]. Moreover, NW arrays have significantly smaller optical reflectance and enhanced light absorption in comparison to thin films [10, 11]. Due to the strain accommodation at the nanowire sidewalls, NWs are less restricted by lattice mismatch, which provides greater freedom in the bandgap engineering and the substrate selection . In most cases, NWs arrays can be formed via the vapor–liquid–solid growth mechanism  using different epitaxial techniques, such as metal organic chemical vapor deposition or molecular beam epitaxy (MBE). One of the most important features of MBE growth is the ability to precisely control the shape, the height, the diameter and the surface density by an appropriate choice of technological parameters by exploring the diffusion-induced growth mode [14, 15], as well as to monitor the formation and time evolution of NWs in situ by the reflection high-energy electron diffraction (RHEED) technique . PV properties of NWs have been investigated for different semiconductor combinations, like Si axial and core–shell single NWs , GaAs core–shell single NW and NW arrays [18, 19], and InAs/Si(111) NW arrays . For the GaAs-based NW arrays, the best conversion efficiency has been demonstrated at the level of 1% at room temperature . In this note, we report on the growth of Au-assisted GaAs p-type-doped NW arrays on the n-type GaAs(111)B substrate and their photovoltaic properties at different substrate temperatures. In our case, the highest efficiency of 1.65% is obtained, which is, to the best of our knowledge, higher than the values reported in the literature for the GaAs NW arrays.
Growth experiments are carried out in EP1203 MBE reactor equipped with the effusion Au cell, on GaAs(111)B n-type (n = 2 × 1018 cm −3) substrates. After the desorption of an oxide layer in the MBE growth chamber, ~100-nm thick GaAs Si-doped (n = 2 × 1018 cm −3) buffer layer is grown on the GaAs(111)B substrate. To ensure n-type doping of the buffer layer, a separate sample has been grown on the GaAs(111)B semi-insulating substrate at the same growth conditions as used for the NWs samples. Electrical measurements (van der Pauw method) confirm n-type doping of the layer with the carrier concentration n = 2 × 1018 cm −3 at the Si cell temperature of 1100 °C. To promote the NW formation by the growth catalyst, the deposition of 0.3-nm thick Au layer is performed at 550 °C. The samples are then kept for 1 min at the same temperature in order to form liquid drops of alloy of Au with the semiconductor material of the substrate. The MBE growth of GaAs Be-doped (P = 1 × 1018 cm −3, as measured from a planar layer) NWs is carried out by the conventional MBE at desired substrate temperature (520–580 °C) with a GaAs growth rate 1 monolayer (ML)/s. The process of NW formation is monitored in situ by RHEED technique. The nucleation of NWs is normally detected after 15 s. The transition to the wurtzite phase is typically observed after the deposition of ~30 ML of GaAs, which is consistent with earlier results for pure GaAs NWs . Total NW growth time is set at 12 min. Four samples are grown at different substrate temperatures, resulting in different morphological properties. The final NW height varies from 1.7 to 2.2 μm, the NWs surface density amounts to 5 × 108–1 × 109 cm −2, and the average diameter is typically 50 nm.
Results and Discussion
I–V characteristics are measured using a Keithley 238 source meter. The samples are placed on a copper base from backside; a metallic sharp tip (D = 0.5 mm) is used as top contact to the NWs/PMMA array. The energy conversion efficiency is determined by illuminating the structures using a halogen arc-lamp with the calibrated power density of P = 100 mW/cm 2.
where VOC and JSC are the open-circuit voltage and the short-circuit current;VM and JM are the voltage and the current density at the maximum power output, respectively, and P is the incident optical power density from the lamp.
In Fig. 2b, we present the J–V characteristics of the best sample (with the highest NWs) in dark and under illumination. Upon illumination of the front surface with the light, the structure yields an obvious photocurrent. The short-circuit current density JSC equals 27.4 mA/cm 2, and the open-circuit voltage VOC amounts to 0.245 V. This corresponds to the conversion efficiency of 1.65% and the fill factor of 25%.
Morphological characteristics and conversion efficiencies of different samples
NWs surface density (cm −2)
NWs height (μm)
Conversion efficiency (%)
1 × 109
1 × 109
1 × 109
5 × 108
To conclude, we have investigated the PV properties of p-doped GaAs NWs array grown on the n-GaAs(111)B substrate at different substrate temperatures. The highest conversation efficiency 1.65% is achieved at 550 °C. To the best of our knowledge, this is the highest value obtained for the GaAs-based NW arrays.
The authors are grateful to the financial support received from RFBR grants, scientific grant of St.-Petersburg Scientific Centre and different scientific programs of RAS. The authors wish to thank Dr. O. Bratenshtain for helpful discussions and providing measurement set-up.
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