Here is the JSON representation you requested:

``` json:

{
  "amorphous carbon PECVD": {
    "CH4": {
      "description": "Methane gas",
      "dep rate": {
        "increase": 8
      }
    },
    "C2H2": {
      "description": "Acetylene gas",
      "dep rate": {
        "increase": 7
      }
    },
    "C2H4": {
      "description": "Ethylene gas",
      "dep rate": {
        "increase": 6
      }
    },
    "C3H8": {
      "description": "Propane gas",
      "dep rate": {
        "increase": 5
      }
    },
    "Ar": {
      "description": "Argon gas",
      "dep rate": {
        "decrease": 2
      }
    },
    "H2": {
      "description": "Hydrogen gas",
      "dep rate": {
        "increase": 7
      }
    },
    "N2": {
      "description": "Nitrogen gas",
      "dep rate": {
        "decrease": 4
      }
    },
    "O2": {
      "description": "Oxygen gas",
      "dep rate": {
        "decrease": 8
      }
    }
  }
}

```

Note that the "dep rate" values are subjective and based on general trends in the literature, and not necessarily indicative of specific conditions or materials. These values should be used as a rough guide only and should not replace actual experimentation and optimization of process conditions.

Here are some sources for more information:
https://www.osti.gov/servlets/purl/1573139
https://www.researchgate.net/publication/229348700_Amorphous_hydrogenated_carbon_films_deposited_by_PECVD_Influence_of_the_substrate_temperature_on_film_growth_and_microstructure
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