The effect of temperature on the resonant tunneling and electric field domain formation in multiple quantum well superlattices

Abstract

Analyzing the photocurrent spectra and the I -V characteristics of weakly coupled GaAs/AlGaAs multiquantum well structures, different transport regimes are distinguished. At low temperatures ͑below ϳ50 K͒, due to the electron coherence over a few periods of the superlattice, electron transport is dominated by sequential resonant tunneling. At higher temperatures, evidences for the increased contribution of nonresonant transport processes, and the subsequent modification in the electric field distribution in the device, are presented. © 1997 American Institute of Physics. ͓S0021-8979͑97͒04504-0͔Electron transport in multiple quantum well ͑MQW͒ superlattices is dominated by sequential resonant tunneling when resonant coupling between adjacent wells exists. This is the origin of a negative differential resistance mechanism whose attendant instabilities give rise to electric field domain ͑EFD͒ formation in the MQW region.1-3 Esaki and Chang 1 first studied this effect in a strongly coupled MQW structure. They observed discontinuities in the I -V characteristics with an average separation equal to the subband spacings ͑in volts͒ in each quantum well ͑QW͒. 1,3 This was explained through the formation of high and low EFDs in the structure and the stepwise increase in the high field domain ͑HFD͒ size with increasing external bias.Even though it is the electron coherence over distances of at least the order of neighboring well separation which is the origin of resonant tunneling, it is difficult to directly relate the dc transport experiments to the electron coherence length. Recently, the sensitivity of the photocurrent spectra in single-bound state MQW structures to well spacings and to the electric field was used to investigate electron coherence effects and the electric field distribution in the device. 4 In this letter, we report on the effect of the temperature on current transport in MQW devices, showing how the transport changes from being dominated by sequential resonant tunneling at low temperatures to domination by nonresonant processes at higher temperatures. Evidences for these nonresonant transport mechanisms which lead to strong changes in the electric field distribution in the device are also presented.The sample used for this study was grown by molecular beam epitaxy on a ͑100͒ semi-insulating GaAs substrate. It consists of 50 periods of 4 nm GaAs wells, uniformly doped with Si to nϭ2ϫ10 18 cm Ϫ3, separated by 20 nm Al 0.22 Ga 0.78 As barriers. As discussed in Ref. 4, the sample was designed to have one bound state in the well, and a first resonant state highly in the continuum, about 24 meV above the barrier. Devices with 200 m diameter were fabricated by standard photolithography and wet chemical etching techniques. The peak at ϳ155 meV in the photocurrent spectrum at V bias ϭϪ2 V and Tϭ10 K, as shown in Fig. 1͑a͒, corresponds to the transition from the ground state to the first continuum resonant state of an isolated QW. The auxiliary peak at ϳ187 meV is due to electron interference effects over a few...

Non-linear dynamics of semiconductor superlattices

“…The previous description becomes explicit in the limit δ 1. In such a case, G + = O(1/δ) and G − = −1 with c − = J from equations (150) and (151). The pulse is approximately a straight isosceles triangle with height (F max − F 1 ) ∼ F max because F max 1.…”

“…As a result, static EFD formation is much more difficult to observe at room temperature than at liquid He or liquid N 2 temperatures. Kastrup [164] showed that the characteristic branches of static EFD formation disappear in a moderately doped GaAs/AlAs SL for temperatures above about 100 K. Xu et al [151] demonstrated that for a strongly doped GaAs/(Al,Ga)As multiple-quantum-well structure the formation of static EFDs is quenched above about 50 K. Wang et al [165] and Li et al [166] performed a detailed study of the temperature dependence of the I -V characteristics for a moderately doped GaAs/AlAs SL. In the investigation by Wang et al [165], there is a strong change in the shape of the I -V curves between 140 and 145 K. The fine structure of the I -V characteristics completely disappears above 156 K. At the same time, current self-oscillations as discussed in section 5 are already observed at 145 K so that a transition from static to dynamic EFD formation takes place between 140 and 156 K. In the investigation by Wang et al [167] and Li et al [166], who apparently used an identical structure, the fine structure in the I -V characteristics disappears above 170 K, and current self-oscillations appear for temperatures of 180 K and higher.…”

“…Once c ± are known as functions of J and F max , this equation determines F max as a function of J and the bias, φ. From equations (151) and 155, we obtain the functions…”

“…However, in the investigations by Helm et al [126,130], Stoklitskii et al [148], Mityagin and Murzin [149], Mityagin et al [76] and Rasulova et al [150], the wells were extremely wide, so that EFDs were formed due to resonant coupling not only between the first and the second, third and fourth subbands, but also between the first and the fifth, sixth, and in some cases even the seventh subbands. Xu et al [151] and Schneider et al [152] reported the observation of EFDs due to the coupling between a bound and a continuum state in n-doped multiple quantum wells with only a single bound state inside the wells. Han et al [153] investigated the formation of EFDs in a graded-gap SL, i.e.…”

Temperature dependence of current self-oscillations and electric-field domains in sequential- tunneling doped superlattices

“…Despite its practical and theoretical interest, the effect of temperature on electric-field domains [20][21][22] and current selfoscillations is still poorly understood. Early numerical calculations were performed with a fixed drift velocity corresponding to a fixed temperature.…”