Dependence of the electrical and optical properties on growth interruption in AlAs/In0.53Ga0.47As/InAs resonant tunneling diodes
© Zhang et al; licensee Springer. 2011
Received: 8 September 2011
Accepted: 23 November 2011
Published: 23 November 2011
The dependence of interface roughness of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunneling diodes [RTDs] grown by molecular beam epitaxy on interruption time was studied by current-voltage [I-V] characteristics, photoluminescence [PL] spectroscopy, and transmission electron microscopy [TEM]. We have observed that a splitting in the quantum-well PL due to island formation in the quantum well is sensitive to growth interruption at the AlAs/In0.53Ga0.47As interfaces. TEM images also show flatter interfaces with a few islands which only occur by applying an optimum value of interruption time. The symmetry of I-V characteristics of RTDs with PL and TEM results is consistent because tunneling current is highly dependent on barrier thickness and interface roughness.
The terahertz [THz] frequency range is receiving considerable attention recently due to various applications. Resonant tunneling diodes [RTDs] have been considered as one of the promising candidates [1–3] for the compact and coherent solid-state THz source at room temperature. Similar to all other quantum-effect devices, one of the most challenging tasks in fabricating RTDs is the control of interface roughness in the quantum-well region. Interface roughness has often been discussed as a factor in determining current through RTDs. Monolayer [ML] fluctuations in tunnel barrier thickness are expected to be important because it is necessary to use tunnel barriers only a few ML thick to obtain the high-peak current densities required for high-speed devices. There have been many theoretical discussions on the importance of surface roughness and interface scattering mechanisms that contribute to peak and valley currents in (In, Ga)As/AlGaAs RTDs and AlSb/InAs RITDs [4–6]. On the experimental sides, there have been reports aimed at linking barrier roughness measured by scanning tunneling microscopy or photoluminescence [PL] spectrum [7, 8]. Therefore, a deeper understanding of the structural imperfections in RTDs grown by molecular beam epitaxy [MBE] and their effects on the electrical characteristics of RTDs is needed in order to reproducibly obtain such RTDs.
In this paper, we report the interface roughness and electrical characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs RTDs' dependence on growth interruption. Though the electrical properties showed anomalous growth interruption dependence, we found an optimum growth condition for increasing the current-voltage [I-V] characteristics' symmetry of RTDs. We also examined the relation between electrical properties and structural imperfections in RTDs. The competition relationship in a PL-integrated intensity of excitonic transitions reveals that the PL spectrum is sensitive to quantum-well-width fluctuation. In addition, transmission electron microscopy [TEM] images also show flatter AlAs/In0.53Ga0.47As interfaces with a few islands which only occur by applying an optimum value of interruption time.
Results and discussion
With respect to the study of PL spectra of different interruption times at the AlAs/In0.53Ga0.47As interfaces, we present the following explanation and discussion: (1) In contrast to high-energy peak B, the presence of peak A and the ratio of I PL (A) and I PL (B) are more sensitive to interruption time at the AlAs/In0.53Ga0.47As interfaces; however, the interruption time is closely related to interface roughness . These phenomena could be explained by considering AlAs/In0.53Ga0.47As interface problems, such as well-size fluctuations in the atomic layer scale, rather than impurity effects since a low-energy peak is not observed in samples with an interruption time of 40 s. (2) As reported in a previous work , with increasing interruption time at interfaces, sufficient time allows cation to migrate and smooth the interfaces, whereas with increasing interruption time beyond 60 s, we found that peak A reemerges in the PL spectra. Therefore, we believe that interface smoothness of a long interruption is commonly better than that of a short interruption. However, this is not the best choice for strained AlAs/In0.53Ga0.47As/InAs RTDs. Then, there must be another significant contribution to the degradation of interface smoothness for a longer interruption time. Normally, after the InAs sub-well was grown, we increased the substrate temperature for the growth preparation of the top In0.53Ga0.47As well layer. The lattice mismatch between In0.53Ga0.47As and InAs is 3.1%, which is considerably large. So with a too long interruption time, which means an in situ anneal, a strain-induced roughness in a highly strained In0.53Ga0.47As layer might play an important role on interface smoothness. In addition, according to the observation of an RHEED pattern, when the interruption time exceeded 60 s, streak pattern of the In0.53Ga0.47As layer was weaker than before. Therefore, strain-induced roughness is probably viewed as the main cause of abnormal PL spectra.
In this work, we have shown the dependence of interface roughness on interruption time by I-V characteristics, PL, and TEM. From the analysis of PL spectra and TEM, we have determined that there is an optimum value of interruption time at interfaces for the growth of highly strained AlAs/In0.53Ga0.47As/InAs RTDs rather than adopting too long interruption times. The results are very useful to further develop the applications of InP-based RTDs.
This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. ISCAS2009T04), the National Natural Science Foundation of China (Grant No. 60876043), the Beijing Natural Science Foundation (Grant No. 2112040), and the Beijing Nova Program (Grant No. 2010B056).
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