Why am I getting this oscilloscope reading? (Half wave rectifier)
Your vertical scale is 20mV-per-division, which is FAR too sensitive. Try a scale voltage of 5V-per-division.
Note the zero-volt symbol at lower left. It is pointing down, indicating that zero volts is far below the screen. It should be pointing right. Such a large DC offset as this could very well overload the linear range of the oscilloscope's amplifiers...I wouldn't trust what you see displayed.