An ALD-Grown Al-Doped Zinc Oxide/n-Si Isotype Heterojunction for Solar Water Splitting

It is well known that zinc oxide (ZnO) films are good passivation layers to semiconductor solar cells. Doped ZnO also serves as an excellent transparent electrode material. The efficiency of many solar cells has been enhanced by adopting this oxide material in the device structures. Besides, ZnO can be prepared by several large-area growth methods such as sputtering, evaporation, CVD, sol-gel, spray, ALD, etc. This benefits the use of ZnO in solar applications. In addition to being a transparent passivation layer or conducting electrode, n-type doped ZnO thin film can directly form an anisotype heterojunction with p-type silicon (p-Si) or an isotype heterojunction with n-type silicon (n-Si). These heterojunctions function simply as solar cells. When converting light power to electrical power, compared with conventional semiconductor p-n junctions, the ZnO/Si heterojunctions show no power loss at the near-surface region in the devices if the incident light comes from the ZnO side, because the ZnO films are transparent to visible light. This advantage further extends the application of ZnO to the active region of devices. In particular, the current flow in the n-ZnO/n-Si heterojunction should be easier since it involves only majority carriers. While numerous studies on the n-ZnO/p-Si anisotype heterojunction were conducted, much fewer works on the n-ZnO/n-Si isotype heterojunction can be found in the literature. In this work, an aluminum-doped ZnO (AZO) thin film was grown on an n-Si substrate by atomic layer deposition (ALD) with in-situ doping technique to form an isotype heterojunction. And a self-powered solar water splitter formed by serially connecting several of such heterojunctions was demonstrated.<br/>The device was fabricated by first using the ALD system to grow a 53 nm thick AZO film on a 0.685 mm thick n-Si (2-7 Ω-cm resistivity) substrate. The growth temperature was low and set at 280 ^oC for best (002) crystallinity of the AZO film. The AZO film thickness was designed for minimum visible light reflection. The aluminum doping was performed by using an in-situ doping technique which involves the introduction of the Al₂(CH₃)₆ (TMA) precursor during the deposition process, which helps precise film thickness control. The resultant AZO film was characterized by Hall measurement to have a low resistivity of 2.48 x 10^-3 Ω-cm and a high electron concentration of 1.3 x 10²⁰ cm^-3. Therefore, the fabricated AZO/n-Si junction behaves like a Schottky junction, and the AZO itself can be the anode contact. TiN was coated at the backside of the Si substrate as the cathode. When serially connecting several devices, copper tape was used as the interposer.<br/>The rectifying dark I-V curve of the AZO/n-Si junction was measured, from which the Schottky barrier height was extracted as 0.632 eV. When illuminated by the light from a tungsten-halogen lamp with an irradiance of 100 mW/cm², the open-circuit photovoltage was measured to be 0.38 V and the short-circuit current density was about 1 mA/cm². To demonstrate the usefulness of the AZO/n-Si isotype heterojunction device, we tried to use it to split DI water under illumination. By using the linear sweep voltammetry (LSV), it was found that the threshold voltage of DI water splitting for the fabricated AZO and TiN electrodes is 1.6 V. This indicates that at least 5 fabricated AZO/n-Si junctions must be connected in series to initiate splitting DI water under illumination. Such experiment has been carried out and the production of hydrogen gas was successfully observed.