# Presentation: 390779

## Passivation of Ge through Boron Carbonitride CVD Films

**John G. Ekerdt, University of Texas at Austin, ****DMR 0603004**

- Deconvolution of the Ge 2*p* XPS signals allows the percentage of oxidation of the Ge substrate to be quantified.

**BCN films are being explored as a potential passivation layer for single crystal Ge surfaces and Ge nanowires.  BCN films are deposited by thermal CVD using dimethylamine borane with NH****3**** and C****2****H****4****.  ISS and XPS were used to determine the minimum BCN thickness resulting in continuous films on Si(100), SiO****2****, and Ge(100).  Due to the high surface sensitivity of ISS, a continuous BCN film should result in complete attenuation of the substrate signal.  BCN becomes continuous at ~2 nm on Si and SiO****2**** substrates, and at ~3 nm on Ge substrates.  XPS was used to determine BCN-coated germanium’s ability to withstand oxidation by monitoring changes in the Ge 2*****p***** and Ge 3*****d***** oxidation states with ambient exposure time.  Discontinuous BCN films delay but do not prevent Ge oxidation. However, a continuous 3.5 nm BCN film shows no oxidation of Ge following 2 weeks of ambient exposure.**

- BCN thickness measured by XPS peak attenuation (Si, SiO2) and ellipsometry (Ge)

- The GeO and GeO2 contributions to overall signal intensity result in 43% GeOx in this representative Ge 2*p* XPS spectrum.

**Notes:**
Motivation:
As electronic device dimensions are scaled down Ge is receiving considerable attention as an alternative to Si. The primary motivation is the higher charge carrier mobility in Ge compared to Si. However, the Ge/GeOx interface is chemically and electrically unstable.  A suitable passivation layer must provide resistance to oxidation without degrading the electrical properties of the semiconductor.  BCN appears to be a promising passivation candidate because it is a non-oxidic ceramic that has been shown in literature to form a smooth interfacial layer with Si(100). The elemental constituents in BCN can also bond directly to Si or Ge so this may minimize interface trap states. To effectively prevent Ge oxidation the coating of BCN must be continuous. This motivates the study of conditions that result in BCN film continuity. Bonding at the interface is also under investigation as are studies of the electrical properties of the interface.

Research details:
 Adjustments to the gas composition during CVD make it possible to tune the BCN stoichiometric composition, constituent bonding, and dielectric constant.
 Si(100) was chosen to develop the experimental protocol because of its well-documented surface chemistry.
 To determine film continuity using ISS, the ratio of the post-BCN substrate signal (either Si or Ge) to the predeposition bare substrate signal was plotted as a function of BCN thickness (determined by XPS peak attenuation for Si and spectroscopic ellipsometry for Ge). When the post-BCN substrate signal disappears entirely, the BCN is assumed to be continuous.
 To explore the oxidation resistance of BCN-coated Ge, BCN was deposited on oxide-free Ge(100).  The samples were then exposed to ambient for extended lengths of time and periodic XPS scans were taken to look for Ge-O bonding in the Ge 2p and Ge 3d signals. The signals were deconvoluted into Ge, GeO, and GeO2 contributions. The % GeOx was then estimated by dividing the GeO + GeO2 peak areas by the sum of all three peak contributions. For BCN films > 2.5nm the Ge 2p signal becomes too noisy to deconvolute and the less surface sensitive Ge 3d must be used to quantify % GeOx.

Future work:
An investigation into the electrical properties of BCN-passivated Ge and GaAs structures is planned.  This will include the fabrication of TaN/HfO2/BCN/Ge (or GaAs) MOSCAP structures for C-V testing in order to estimate interface state densities. These densities will be compared to unpassivated TaN/HfO2/Ge (or GaAs) MOSCAPs.

## Design, Growth, and Properties of Boron-Based Thin Films for Electronics and Nanosized Electronics

**     **As the dimensions of electronic devices are scaled, attention is being given to Ge and GaAs as alternatives to Si; the primary motivation is the higher mobility of holes and electrons in Ge and GaAs. Nanowires of Ge and GaAs materials are intriguing candidates for nanoscale integrated circuits or sensing devices. However, without a stable interface between the Ge or GaAs semiconductor and an oxidizing environment, the advantages are lost.  BCN holds promise to passivate Ge and GaAs. We have shown that continuous BCN coatings can prevent Ge oxidation, providing the motivation to explore the more general use of BCN. Early electrical test results indicate that MOSCAP structures fabricated from BCN-passivated GaAs exhibit significantly less leakage current than similar unpassivated structures. Also, TEM images of BCN deposited on silicon nanowires demonstrate conformal coatings.
-      This grant has provided the opportunity for three undergraduate students, Eric Teletzke, Tiffany Wu, and Michael Qian, to work in the lab gaining exposure to surface science and materials research.

**Design, Growth, and Properties of Boron-Based Thin Films for Electronics and Nanosized Electronics**
**John G. Ekerdt, University of Texas at Austin, ****DMR 0603004**

**Notes:**
MOSCAP: metal-oxide-semiconductor capacitor
TEM: transmission electron microscopy