Can Zener or Schottky diodes be used as a substitute for varicap diodes in tuning a VCO?
Can Zener or Schottky diodes be used as a substitute for varicap diodes in tuning a VCO?

Yes they can, depending on the circumstances.

A varicap diode is similar to a "normal" silicon diode, however the diode is designed such that the junction capacitance under reverse biased conditions is more predictable, has a more linear relation to the reverse voltage and sometimes also has a larger value.

A Zener diode is similar to a "normal" silicon diode, however the reverse breakdown voltage has a more predictable value. Besides that, it also has a reverse biased junction capacitance just like any other diode.

A Schottky diode also has a reverse biased junction capacitance which is also dependent on the reverse voltage.

So all three diodes (normal, Zener, Schottky) have "varicap" behavior so can in principle be used.

However, the varicap is better optimized for the task, see my paragraph about the varicap above.

Also, the Zener and Schottky diodes have some disadvantages which may render then useless in some applications.

They often have a higher leakage current. Especially the Schottky diodes have high leakage at high temperatures.

They might be more noisy. Especially the Zener diodes are known to be noisy (inject noise into your circuit).

They might have a capacitance with a lower quality factor. This is due to the series resistance of the diode's capacitance. Such a series resistance might lower the Quality factor of your tuning circuit or oscillator tank. This might lead to lack of loopgain in your oscillator so the oscillator will stop oscillating. The series resistance might also inject additional noise.

So yes, possible but there are some "gotchas" you have to be aware of.