Gas bubbles and blisters in thin film cadmium selenide deposited by magnetron sputtering

Abstract

Cadmium Selenide (CdSe) is a source of Selenium in CdSeTe/CdTe photovoltaic device fabrication. CdSe thin films have been deposited using pulsed DC magnetron sputtering. The thin films are high quality, compact with the columnar structure usually associated with sputtered thin films. The thin films exhibit mixed hexagonal and cubic phases due to the presence of stacking faults. Argon is observed to be present in the as-deposited films, implanted during film growth from the sputter-working gas. The thin films have been exposed to high temperature annealing treatments and to cadmium chloride (CdCl₂) activation to assess microstructural changes that occur. After annealing, voids appear in the film caused by the growth of argon gas bubbles. This leads to blistering on the CdSe surface. The CdCl₂ treatment causes even more serious void formation and surface blistering. Similar problems have been observed with sputtered CdTe thin films. Argon gas atoms are trapped in the growing films by stacking faults. When the stacking faults are removed by the high temperature treatments, the argon diffuses into gas bubbles. These bubbles continue to expand causing serious voiding and catastrophic blistering. Although, the phenomenon can be mitigated by increasing the substrate temperature and by reducing the deposition rate, argon gas bubbles are still formed. Voiding is a serious problem in photovoltaic device fabrication, but the problem can be resolved by using an evaporative deposition technique to avoid argon implantation. The use of sputter-deposition of CdTe and CdSe should be avoided in the fabrication of high efficiency photovoltaic devices.