Recently, Hickmott et al. l reported a remarkable oscillatory structure with period eAF=#co L o, the longitudinal-optic phonon energy, in the lowtemperature reverse-bias J(V) characteristics of « + GaAs/(AlGa)As/A?~GaAs//7 + GaAs devices. The structure was observed only in magnetic fields (B) above 4 T which, they claimed, reduced ionizedimpurity scattering by inducing freezeout onto shallow donors. This permitted sequential ballistic acceleration of electrons up to the LO phonon emission threshold.We have studied similar molecular-beamepitaxy-grown, 200-/xm-diam mesas with layer thicknesses and doping as follows: substrate and buffer, 200 /zm, 2xl0 18 cm -3 ; n~ layer, l o ^m, 1 x 10 15 cm*" 3 ; Alo.35Gao.55As barrier, 168 ± 10 A, undoped; top layer, 1 /xm, 2x 10 18 cm -3 . Figure 1 shows typical J(V) plots. Mesas of type A have J( V) of similar overall form to those in Ref. 1, but of much larger amplitude ( -10 3 larger at K= -0.1 V). Mesas of type B have an additional low-bias (V > -0.2 V) contribution to J which may arise from imperfections (e.g., microchannels) in the (AlGa)As barrier. Even at 5 = 0, derivative plots of both mesa types show oscillatory structure (Fig. 2) of comparable relative amplitude (e.g., ~ 2% at -0.2 V) to that in Ref.1. The periodicity of the main peaks is#o> LO /e-Structure is clearly visible (Fig. 2, curves c and d) even at temperatures (30 K) at which the majority of donors are ionized. The weaker intermediate extrema, seen only for T < 4 K, probably arise from higher harmonic structure in J( V)\ at 4 K some peaks in d 2 J/dV 2 are only 2 mV wide. Note that our barriers are -~ 20% narrower than those in Ref. 1 and that J is exponentially sensitive 2 to barrier height and width. Calculation shows that J(V) of Fig. 1 (type A) is fully consistent with the barrier-tunneling characteristics. The 0-6 0-4 0-2 Reverse Bias [V] FIG. 1. Log/vs -Kfor two of our devices. 0-1 0-2 0-3 Reverse Bias [V] FIG. 2. d 2 J/dV 2 vs -V: Curves a and d, type-A mesas; curves b and c, type-B mesas.larger J(V) passed by our devices may be crucial for observation of oscillatory structure even at B = 0 and up to 30 K and for understanding of its origin. 3,4 The low reverse-bias shoulder on J ( V) in type-B mesas is removed by modest (~ 5 T) transverse magnetic fields (/.Li?), suggesting that it may be due to microchannels in the barrier. 5 However, oscillations for V > 0 are not observed. 5 Oscillatory structure in both mesa types for V < -0.2 V is essentially unaffected by magnetic fields (II and LJ) up to 12 T.

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