Different Zener Diode Symbols

The base symbol I can fine no definite standard reference for. I'd guess the majority of us would use it.

^{simulate this circuit – Schematic created using CircuitLab}

Graphical samples for the Australian Standard AS 1102 (1989) Graphical symbols for electrotechnical does appear to reference this symbol. I don't have a copy, so I cannot confirm this.

Europe: International Electrotechnical Commission IEC60617 (2012) Graphical Symbols for Diagrams provide the following:

with the IEC60617 (1996) version defining the bars. for:

• Schottky
• Tunnel
• Zener
• Bi-directional
• Unitunnel

Their usage of straight bars (05-02-04) is for Bi-directional or breakdown diode.

US: IEEE 315 (1975) Standard for Graphic Symbols for Electrical and Electronics Diagrams provides the straight bar symbol:

ANSI Y32.2 (1975) Graphic Symbols for Electrical and Electronics Diagrams has been superseded by IEEE315. It references the same symbol as IEEE315.