I can't load in fastboot mode

Hello,
I would like to root my Z5C, but I can't load the phone in fastboot mode.
I tried the following :

• adb reboot bootloader (with my phone one, with usb debug)

  the phone shut down completely, but nothing is displayed on the screen

• power up + volume up (from my phone shutdowned)

  there are three vibrations, and sometimes the logo Xperia appears for a few second, but then the screen show the charging icon

• power down + plug usb (from my phone shutdowned)

  nothing appends

each time, I tried the following command to check if the phone was indeed in fastboot mode :
fastboot devices
but I get nothing (empty list).
Could you help me please?
thanks in advance.

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