Diode-like characteristics ofnanometer-scale semiconductor channels with a broken symmetry

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1 Available online at Physica E (4) 6 Diode-like characteristics ofnanometer-scale semiconductor channels with a broken symmetry A.M. Song a;, I. Maximov b, M. Missous a, W. Seifert b a Department of Electrical Engineering and Electronics, UMIST, Manchester M6 QD, UK b Solid State Physics and Nanometer Consortium, Lund University, Lund, Sweden Abstract We present a new type ofnanometer-scale semiconductor nonlinear device, called self-switching device (SSD). The device was realized by simply etching insulating grooves into a semiconductor, between which a narrow channel with a broken symmetry was formed. Because of the asymmetry in the channel boundary, an applied voltage V not only changes the potential prole along the channel direction, but also either widens or narrows the eective channel width depending on the sign of V. This results in a diode-like current voltage characteristic but without the use ofany doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually to more than V by simply adjusting the channel width. Furthermore, only one lithography step was needed to fabricate SSDs. We used two dierent material systems, InGaAs InP and InGaAs InAlAs, to realize SSDs and the results at room temperature were compared. We also show that by adding a third terminal to an SSD as a gate, the turn-on voltage ofthe device could be tuned by the gate bias and the device functions either as a tunable diode or as a transistor.? 3 Elsevier B.V. All rights reserved. PACS: 73.5.Fq; 73.4.Ei; Be; 73.4.Kp Keywords: Nanodevice; Nonlinear; Symmetry; InGaAs-InP; InGaAs-InAlAs Great eorts have been made to realize novel nanoelectronic devices for the future generations ofintegrated circuits. At the nanometer scale, new transistor concepts, such as single-electron transistor (SET) [ 3], and lateral-gate transistor [4 7] have been invented. An SET possesses remarkable properties such as low-power consumption and ultra-high sensitivity, while the lateral-gate transistor has very simple design and requires only one-step lithography. Recently, dierent approaches have been carried out to fabricate SETs for room-temperature Corresponding author. Tel.: ; fax: address: a.song@umist.ac.uk (A.M. Song). and high-frequency operations [8 ], but the reproducibility ofthe fabrication remains a serious challenge due to the required ultra-small dimension (typically a few nanometer). Apart from such novel transistors, possible nanometer-scale counterparts of conventional diodes were also investigated. Among them are the recently realized ballistic rectier [,] and three-terminal ballistic junction [3 5]. Both types ofdevices have been demonstrated to work at room temperature and at GHz frequencies [6 ]. Nevertheless, the working principle relies on ballistic electron transport, meaning that the device characteristic dimension must be smaller than the electron mean free path. Therefore, although room-temperature ballistic devices have been reproducibly fabricated /$ - see front matter? 3 Elsevier B.V. All rights reserved. doi:.6/j.physe.3..9

2 A.M. Song et al. / Physica E (4) 6 7 using high-mobility III V semiconductors and standard electron-beam lithography, it is still not possible to fabricate room-temperature ballistic devices using silicon materials because ofthe very short mean free path (around nm or less). In this work, by tailoring the boundary ofa narrow semiconductor channel to break its symmetry, we have realized a new type ofnanometer-scale, nonlinear device, called self-switching device (SSD). Because ofthe geometrical asymmetry, the two-terminal device functions like a conventional diode, but the turn-on voltage can be tuned by simply adjusting the width ofthe narrow channel. Furthermore, no barrier structure or doping junction was needed in an SSD, meaning that the device operates with a completely dierent working principle than a conventional diode. Because the required minimum feature size is around 5 nm which is within the limit ofstandard electron beam lithography, we have reproducibly fabricated room-temperature SSDs using both InGaAs InP and InGaAs InAlAs modulation-doped quantum well structures. From the new working principle, which does not require a high electron mobility or long electron mean free path, we also expect SSDs to be reproducibly fabricated using silicon materials by the current advanced CMOS technology. Fig. shows the schematics ofthe InGaAs InP and InGaAs InAlAs modulation-doped quantum well structures, which we used to fabricate SSDs. The strained In :75 Ga :5 As=InP wafers were grown by metal organic vapor phase epitaxy [], while the lattice-matched InGaAs InAlAs wafers were grown by molecular beam epitaxy. At room temperature, the density and mobility ofthe two-dimensional electron gas (DEG) are about 4:7 5 m and : m =Vs, respectively, in the InGaAs InP wafers, and about. :5 6 m and : m =Vs, respectively, in the InGaAs InAlAs quantum well structures. In order to fabricate SSDs using wet etching after the electron-beam lithography, the DEGs in both types ofwafers were designed to be within a short distance from the surface: about 4 6 nm. A scanning electron microscope image ofa typical SSD is shown in Fig. (a), in which the dark lines are the etched insulating grooves into a piece ofthe semiconductor wafers. The continuation of the trenches to the device boundary broke the device symmetry and ensured that the current could ow only via the narrow channel between the grooves. Because ofthe depletion region close to the etched boundaries [gray areas in Fig. (b)], the eective channel is narrower than the physical dimension. As is demonstrated in Figs. (c) and (d), the eective channel width can be either widened or narrowed depending on the sign ofthe applied voltage V. This results in a preferred direction of current ow, hence the diode-like current voltage characteristics. A large number of SSDs were fabricated from both types ofwafers and the results were very similar. The I V characteristic of an SSD, fabricated from an InGaAs InP wafer and measured at temperature T = 4: K is shown in Fig. 3(a). The channel width W = 8 nm was optimized to have a zero turn-on voltage. In this case, at V = the channel was just about to pinch o, namely W =8nm is about twice the depletion length, so that ifa positive voltage is applied, the channel will be opened electrically whereas the channel will be further InGaAs-InP wafer Undoped InP nm ***************************** Undoped InP nm Undoped InGaAs 9nm Undoped InP buffer Undoped InP substrate δ doping DEG InGaAs-InAlAs water UndopedInGaAs nm Undoped InAlAs 3 nm ***************************** Undoped InAlAs nm Undoped InGaAs 8nm Undoped InAlAs buffer Undoped InP substrate Fig.. Schematics ofthe InGaAs InP and InGaAs InAlAs modulation-doped quantum well structures, which were used to fabricate SSDs.

3 8 A.M. Song et al. / Physica E (4) 6 µm.4.3 T=4. K (a) Depletion.. InGaAs-InP W=8 nm, L=. µm (b) V= (a) (c) (d) V>, channel open V>, channel open Fig.. (a) A scanning electron micrograph ofa self-switching device. (b c) schematically show the changes in the depletion regions in the device under dierent bias conditions, indicating how the device functions like a diode even though no barrier structure or any doping junction was used. depleted ifa negative voltage is applied. Similarly, for devices with a smaller channel width W, we found that the turn-on voltage was larger than zero, and if W 8 nm current can always ow as long as V []. This shows that not only the working principle ofthe nonlinear characteristics is completely dierent from a conventional diode for being not based on any doping junction or barrier structure, but also the turn-on voltage can be conveniently tuned by simply adjusting the channel width. The length of the channel, L, was also found to inuence the (b).5.5 InGaAs-InP T=4 K T=3 K W=6 nm, L=. µm Fig. 3. (a) The nearly ideal I V characteristic ofan InGaAs InP based device with the channel width W = 8 nm and length L =: m measured at T =4: K. (b) The I V characteristics ofan SSD with a narrower channel, measured at 4 K and room temperature. device property. The experiments revealed that a certain length is chosen to have a well-dened diode-like characteristic. For most ofour devices, L=: m was needed, which was limited by the wide etched trenches, largely due to the (quasi-)isotropic etch rate ofthe wet chemical etching. From the working principle, we expect that ifthe width ofthe etched trenches is reduced, the length ofthe channel can be decreased, too, since the narrower gap ofthe trenches would lead to a more pronounced self-switching

4 A.M. Song et al. / Physica E (4) 6 9 eect on the modulation ofthe eective channel width by the applied bias. The self-switching devices made from the InGaAs InP wafer were found to be temperature sensitive. For example, the device shown in Fig. 3(a) had a less pronounced I V characteristic at room temperature because signicant leakage current arised under the reverse-bias condition. This suggests that the depletion length became shorter at room temperature. To achieve the zero turn-on voltage for higher temperatures, a narrower channel width is needed. Fig. 3(b) is the result ofa device with W = 6 nm, measured at T = 4 K and room temperature. At 4 K, the I V is virtually ideal with a zero turn-on voltage. However, at room temperature, current appeared under the reverse-bias condition. Further experiments showed that even for SSDs with narrower channel widths, the reverse current did not reduce signicantly. By fabricating devices with dierent etching depths, we concluded that there was some leakage between the buer layer and the substrate in the InGaAs InP wafers. Such a problem seems not to exist in the InGaAs InAlAs wafers. As can be seen in Fig. 4, the I V characteristic ofthe SSD, which was fabricated using the InGaAs InAlAs wafer and measured at room temperature, does not display any clear leakage current under the reverse-bias condition. Comparing with Room temperature InGaAs-InAlAs Fig. 4. The I V characteristic ofan SSD fabricated using the InGaAs InAlAs wafer and measured at room temperature. No clear leakage current was observed under the reverse-bias condition T= K InGaAs-InAlAs -5 5 Fig. 5. The I V characteristic ofan InGaAs InAlAs based SSD showing a high turn-on voltage. the InGaAs InP materials, the InGaAs InAlAs wafers also seemed to allow the realization ofa higher turn-on voltage. Fig. 5 plots the I V characteristic ofan SSD with a much narrower channel width. The turn-on voltage under the forward-bias condition is about 7 V, and is more than V under the reverse-bias condition. One ofthe most signicant properties ofthe SSD is the remarkably simple lithography, i.e., only requiring to create insulating lines on a piece ofconducting semiconductor, which is similar to the fabrication ofa lateral gate transistor [4 7]. By combining a few SSDs, simple circuits such as logic gates can be fabricated also in one lithography step []. Actually, by extending one ofthe trenches ofan SSD, an additional gate can be formed, as sketched by the inset in Fig. 6. By tuning the gate voltage V G from :4 to :4 V at a step of: V, the turn-on voltage, shown in the source drain current I SD versus source drain voltage V SD curves, changed from : V to about :7 V. Gain can also be achieved ifthe device is used for amplication. A dierence from a normal transistor is, however, that the transistor characteristics depend on the sign ofthe source drain voltage as shown in Fig. 6. This is a result ofthe existence ofthe self-switching eect in the upper part ofthe device, and such an eect might be useful in compensation of unwanted eects such as characteristic changes due to temperature variations. Therefore, this three-terminal device

5 A.M. Song et al. / Physica E (4) 6 I DS (µa) T=4 K S Gate V G =-.4V, -.6V,..., -.4V V DS (V) Fig. 6. The source drain current as a function of the source drain voltage at dierent gate biases ofa device sketched by the inset. This shows that the device can be regarded as a diode with a tunable turn-on voltage as well as a transistor. combined the functionalities of a self-switching diode with a tunable turn-on voltage and a lateral gate transistor. Such a exibility as well as the simple, one-step lithography may open up more possibilities ofcircuit applications. In conclusion, we have demonstrated a new type ofnanometer-scale diode, realized in only one step oflithography but not based on any doping junction or barrier structure. The turn-on voltage ofthe self-switching device can be widely tuned from virtually to more than V, by simply adjusting the channel width. With an extended design, a three-terminal device was demonstrated to have the functionalities ofboth a diode and a transistor. Although the devices were so far fabricated using only III V heterostructures, we expect that the devices can readily be made using silicon (such as SOI) wafers since the working principle does not require a high electron mobility. Ifthis is true, it will represent a major step closer to possible practical applications ofthe self-switching device. D This work was supported by the Engineering and Physical Science Research Council (EPSRC), Swedish Research Council, and European LTR research project NEAR. We thank P. Omling and L. Samuelson for useful discussions. References [] D.V. Averin, K.K. Likharev, J. Low Temp. Phys. 6 (986) 345. [] T.A. Fulton, G.J. Dolan, Phys. Rev. Lett. 59 (987) 9. [3] U. Meiave, M.A. Kastner, S.J. Wind, Phys. Rev. Lett. 65 (99) 77. [4] A.D. Wieck, K. Ploog, Appl. Phys. Lett. 56 (99) 98. [5] J. Nieder, A.D. Wieck, P. Grambow, H. Lage, D. Heitmann, K. von Klitzing, K. Ploog, Appl. Phys. Lett. 57 (99) 695. [6] J.S. Mclean, A.D. Wieck, M. Bleder, K. Ploog, Appl. Phys. Lett. 6 (99) 34. [7] A.D. Wieck, K. Ploog, Appl. Phys. Lett. 6 (99) 48. [8] H. Ishikuro, T. Fujii, T. Saraya, G. Hashiguchi, T. Hiramoto, T. Ikoma, Appl. Phys. Lett. 68 (996) [9] L. Zhuang, L.J. Guo, S.Y. Chou, Appl. Phys. Lett. 7 (998) 5. [] Yu.A. Pashkin, Y. Nakamura, J.S. Tsai, Appl. Phys. Lett. 76 () 56. [] A.M. Song, A. Lorke, A. Kriele, J.P. Kotthaus, W. Wegscheider, M. Bichler, Phys. Rev. Lett. 8 (998) 383. [] A.M. Song, Phys. Rev. B 59 (999) 986. [3] K. Hieke, M. Ulfward, Phys. Rev. B 6 () 677. [4] H.Q. Xu, Appl. Phys. Lett. 78 () 64; H.Q. Xu, Appl. Phys. Lett. 8 () 853. [5] L. Worschech, B. Weidner, S. Reitzenstein, A. Forchel, Appl. Phys. Lett. 78 () 335. [6] A.M. Song, P. Omling, L. Samuelson, W. Seifert, I. Shorubalko, H. Zirath, Jpn. J. Appl. Phys. 4 () L99. [7] A.M. Song, P. Omling, L. Samuelson, W. Seifert, I. Shorubalko, H. Zirath, Appl. Phys. Lett. 79 () 357. [8] I. Shorubalko, H.Q. Xu, I. Maximov, P. Omling, L. Samuelson, W. Seifert, Appl. Phys. Lett. 79 () 384. [9] I. Shorubalko, H.Q. Xu, I. Maximov, D. Nilson, P. Omling, L. Samuelson, W. Seifert, IEEE Electron Device Lett. 3 () 377. [] L. Worschech, H.Q. Xu, A. Forchel, L. Samuelson, Appl. Phys. Lett. 79 () 387. [] P. Ramvall, N. Carlsson, P. Omling, L. Samuelson, W. Seifert, M. Stolze, Q. Wang, Appl. Phys. Lett. 68 (996). [] A.M. Song, M. Missous, P. Omling, A.R. Peaker, L. Samuelson, W. Seifert, Appl. Phys. Lett. 83 (3) 88.