Surface Termination and Roughness of Ge(100) Cleaned by HF and HCl Solutions

Oxide removal from Ge(100) surfaces treated by HCl and HF solutions with different concentrations are systematically studied by synchrotron radiation photoelectron spectroscopy (SR-PES). SR-PES results show that clean surfaces without any oxide can be obtained after wet chemical cleaning followed by vacuum annealing with a residual carbon contamination of less than 0.02 monolayer. HF etching leads to a hydrogen terminated Ge surface whose hydrogen coverage is a function of the HF concentration. In contrast, HCl etching yields a chlorine terminated surface. Possible etching mechanisms are discussed. Surface roughness after HF and HCl treatments is also investigated by AFM, which shows that HF treatment leaves a rougher surface than HCl. Germanium (Ge) is increasingly being studied for MOSFET applications to take advantage of its high intrinsic electron and hole mobility. To fabricate high performance devices on Ge, it is essential to understand Ge surface chemistry and find an effective way to clean and passivate its surface. Although Si surface cleaning and passivation have been extensively studied, only recently has some research been done on Ge surfaces. Conventional XPS results show that HF etching removes Ge oxide and carbon contamination significantly, and HCl etching leads to a chlorine terminated Ge(111) surface, which only forms Ge monochloride. However, it is difficult to probe the details of the chemical nature of treated surfaces and quantify the surface termination and cleanness with conventional XPS, because of its limited surface sensitivity and resolution. In addition, little attention has been paid to the HF concentration, which turns out to be an important factor in the surface hydrogen passivation. In this work, we study the Ge(100) surfaces treated by aqueous HCl and HF solutions with three different concentrations by synchrotron radiation photoelectron spectroscopy (SR-PES) at Stanford Synchrotron Radiation Laboratory (SSRL). Using SR-PES, we can tune the photon energy to achieve very high surface sensitivity and good resolution, so the chemical states of treated surfaces can be resolved unambiguously, and the surface termination and cleanness can be quantified. We find that HF treatment results in a hydrogen terminated surface, and the hydrogen coverage depends on the HF concentration. In contrast, a Cl terminated Ge(100) surface is achieved after HCl treatment. Both monochloride and dichloride are formed on the surface. The termination difference between HF etching and HCl etching can be explained by the etching mechanism. In both cases, the residual carbon and oxygen after chemical etching can be removed by vacuum annealing