Localization in a non-degenerate two-dimensional electron gas.

We report localization effects in a non-degenerate two-dimensional electron gas formed on a solid hydrogen substrate. Magnetoresistance measurements in a Corbino geometry display an essentially classical positive B sup 2 field dependence on clean surfaces. Lower mobility surfaces, however, are characterized by the appearance of a negative magnetoresistance for fields less than ~ .2Tau, in good agreement with present weak localization theories. On extremely disordered surfaces we find large increases in resistivity but a finite residual electron mobility, micro sub (res), of the order of (h vector e) over (m sub (el) K sub B Tau). We associate this with the strong localization of electrons with energy less than a disorder dependent threshold and an average diffusivity, D ~ (h vector) over (m sub (el)), for those electrons near the threshold.