A Si bistable diode utilizing interband tunneling junctionsX. Zhu, X. Zheng, M. Pak, M. O. Tanner, and K. L. Wang Citation: Applied Physics Letters 71, 2190 (1997); doi: 10.1063/1.119377 View online: http://dx.doi.org/10.1063/1.119377 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of the a - Si Ge : H thickness on the conduction mechanisms of n -amorphous- Si Ge : H ∕ p -crystalline-Si heterojunction diodes J. Appl. Phys. 97, 083710 (2005); 10.1063/1.1866494 Room temperature tunneling transport through Si nanodots in silicon rich silicon nitride Appl. Phys. Lett. 86, 063503 (2005); 10.1063/1.1861129 Universal tunneling behavior in technologically relevant P/N junction diodes J. Appl. Phys. 95, 5800 (2004); 10.1063/1.1699487 Trap-assisted tunneling at temperatures near 77 K in laser annealed Si n + -p junctions J. Appl. Phys. 90, 860 (2001); 10.1063/1.1378330 Vertical integration of a spin dependent tunnel junction with an amorphous Si diode Appl. Phys. Lett. 74, 3893 (1999); 10.1063/1.124215

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A Si bistable diode utilizing interband tunneling junctionsX. Zhu,a) X. Zheng, M. Pak, M. O. Tanner, and K. L. WangDevice Research Laboratory, Department of Electrical Engineering, University of California, Los Angeles,California 90095-1594

~Received 25 April 1997; accepted for publication 26 July 1997!

A forwardS-type bistability was observed in a Si diode with two doubled-doped Si tunnel junctionsbetween thep and n contacts. The conductivity in the two branches of the bistableI –V curvechanges by seven orders of magnitude. This, coupled with the all-silicon nature of the device, makesit a very attractive multistate device for practical applications. The bistability is explained by amechanism, referred to as ‘‘band switching,’’ which is supported by temperature dependence of theI –V characteristics. ©1997 American Institute of Physics.@S0003-6951~97!01339-9#

As first proposed by Esaki and Tsu,1 band engineeringcan be achieved by changing the composition of the consti-tuting semiconductor materials~heterostructures! or bymodulating the doping profile. Heterosuperlattices and dop-ing superlattices are typical examples of the two approaches.In the area of doping superlattices, GaAs doping superlat-tices have been studied for their tunability in conductivity,luminescence, and optical absorption, etc. Recently,d-dopedGaAs structures were also studied for their negative differ-ential resistance2–5 and multistate phenomena. Similar re-sults were also reported in Si based structures.6–9

Previously, we reported bistableI –V characteristics in ad-doped SiGe/Si diode6 and its application as a static randomaccess memory~SRAM! cell.7 In the former letter, a SiGequantum well, together with twod-doped layers were used toachieve bistableI –V characteristics. In this letter, a similarbistability was demonstrated in a band engineered full Sidevice.I –V bistability was accomplished by band engineer-ing with two doubled-doped tunnel junctions. The full Sifeature of the device makes it more compatible with themainstream Si technology. The bistability of the new diodecan be explained with a band switching mechanism. A tem-perature dependence study of theI –V curves is discussed tofurther confirm the mechanism. Different from the previousSiGe bistable diode, the new diode has several positive feed-back processes that lead to a shorter turn-on and turn-offtime, and hence, a faster switching performance. Comparedwith the Shockley diode,10 which is based on diffusion cur-rent, the new diode relies on interband tunneling. Moreover,the I –V characteristics of the new diode are ‘‘harder’’ thanthe Shockley Diode due to Fermi-level pinning in the newdiode, which will be described later.

The sample used in this study was grown by a Perkin–Elmer molecular beam epitaxy~MBE! system on a borondopedp-type Si~100! substrate with a resistivity of 0.02Vcm. A 2000 Åp-type (p;531018/cm3) Si buffer layer wasfirst grown at 550 °C, followed by the first tunnel junctionthat consists of ap-type d-doped layer~p;331013/cm2,achieved by stop growth!, a 40 Å undoped Si layer, and ann-type d-doped layer (n;131014/cm2). The n-typed-doped layer has a thickness of 40 Å and was achieved bythe growth interruption technique.11,12 The sheet concentra-

tions were calculated from the B and Sb fluxes and previousd-doping studies.11,12 After the first junction, an undoped Sispacer layer of 320 Å was grown at 500 °C, followed by thenext junction that is identical to the first junction. Finally, a4000 Å antimony doped (n;531018/cm3) Si contact layerwas grown. Two Sbd-doped layers were inserted right be-neath the surface for better ohmic contact.13 Testing diodeswith an area of 50mm3100mm were fabricated using con-ventional photolithography and reactive ion etching~RIE!.Al was used as both the etching mask and the contact metal.

Figure 1 is the bistableI –V curve of one of our samples,XZ024, as obtained by HP4142 at room temperature usingcurrent mode measurement. The two distinctive branches inthe I –V characteristics are referred to as the off and on statein this letter. The diode exhibits a resistance of about 20 MVin its off state and switches to the on state at about 5.45 V.The on state has a resistance of about 2.3V. Reverse‘‘breakdown’’ occurs at around 1 V.

Shown in Figs. 2~a!–2~d! are the band diagrams underdifferent biases. Figure 2~a! is the band diagram at zero bias~off state!. Regions 1p, 1n, 2p, and 2n are thep- andn-type d-doped layers in junctions 1 and 2, respectively.Each of the twod-doped junctions behaves like an Esakitunnel diode.14 When a small forward bias is applied@seeFig. 2~b!#, most of the bias drops over the middle spacerlayer becaused-doped layers 1n and 2p are reverse biasedand therefore the device exhibits a high resistance. TheFermi levels at thep- andn-d layers are essentially pinnedtogether. The small forward current is composed of the in-

a!The author is currently associated with Rockwell Semiconductor Systems,Inc., Newport Beach, CA.

FIG. 1. I –V curve of XZ024 at room temperature. TheI –V characteristicswas obtained by a HP4142 using current mode measurement.

2190 Appl. Phys. Lett. 71 (15), 13 October 1997 0003-6951/97/71(15)/2190/3/$10.00 © 1997 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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terband tunneling currentI tun through the middle spacer bar-rier layer and the diffusion currentsI th,p , I th,n over the bar-riers. The avalanche process is unlikely because of thenegative temperature coefficient of the switch-on voltage~which will be described later!. With an increased forwardbias, the electric field in the middle spacer layer is enhanced.The tunneling currentI tun increases almost exponentially,i.e., I tun; f (E)* exp(2a/E), wherea is some constants,E isthe electric field, andf (E) is some slow varying function ofE ~compared to the exponential term!. When this interbandtunneling current exceeds the smaller of the two tunnelingcurrentsI 1 and I 2 , carriers will start to accumulate at thejunctions. If we assumeI 1 is smaller, then electrons willaccumulate in layer 1n. This charge accumulation will re-duce the band bending at junction 1 and cause larger holediffusion current across the barrier layer 1n. This thermalcurrent will cause more hole accumulation at layer 2p. Thiswill in turn cause larger electron diffusion current across the2p barrier and cause more electron accumulation at layer1n. A positive feedback is thus formed. On the other hand,with the reduced band bending, the electron/hole quasi Fermilevels at the 1n/1p and 2n/2p junctions will split and thejunction tunneling currentsI 1 andI 2 will decrease due to theintrinsic negative differential resistance characteristics oftunnel junctions. This reduction of the tunneling currentI 1

and I 2 will speed up the charge accumulation. These twopositive feedback processes will continue until the bandswitches to the on-state rapidly@see Fig. 2~c!#. This occursrapidly when the accumulated charge approaches the respec-tive ionized dopant concentrations. Geometrically, this hap-pens whenx15x2 @as shown in Fig. 2~b!#. Therefore, theswitch-on voltageVs can be estimated by

Vs5~Eg /e!~L2d!d,

whereEg is the Si band gap,d is the junction width which isequal to the spacer width between the twod layers in thejunction plus half the width of then-type d layer (40 Å140 Å/2), andL is the middle spacer layer width plus half

the width of then-typed layer (320 Å140 Å/2). The result-ing Vs55.2 V agrees reasonably well with our experimentalresult of 5.45 V.

The transition from the on to the off state occurs at thepoint when the forward current is equal to the larger of therecombination currentsI r1 and I r2 at the 1n/1p and 2n/2pjunctions. If we assumeI r1 is larger, as long as the conduc-tive current can not support the recombination between 1pand 1n, the amount of nonequilibrium electrons accumulatedat layer 1n will decrease and cause the band bending tooccur again at layer 1n. This band bending will restrict thehole injection into layer 2p and destroy the steady state ofthe accumulated holes at layer 2p. As a result, the holespreviously accumulated at layer 2p will be depleted and thusresult in a barrier~for electrons! at layer 2p. This will in turnblock the electron current to layer 1n and a positive feedbackresults to further increase the band bendings. In addition, theincreased band bendings will also increase the recombinationcurrents and enhance the carrier depletion at thed layers,thus further increasing the band bendings. These two positivefeedback loops make the diode switch back to the off staterapidly as shown in Fig. 2~b!. From this mechanism, thetransition from the on to the off state occurs when the ap-plied voltage is around the built-in voltage (Vbi) of the twocontacts or close toEg /e'1.1 V. The observed transitionvoltage is 1.6 V. This is probably due to an extraordinarilylarge recombination current of the tunneling junctions. Inthis situation, extra bias is needed to provide enough conduc-tive current to support the recombination. A fine structure isalso observed in the on state near a current of 32 mA. Themechanism of this fine structure needs further study.

The reverse breakdownlike characteristics at around 1 Vcan also be explained using the band switching mechanism.Figure 2~d! is the band diagram under a reverse bias. For asmall reverse bias, only a very small current can flowthrough the middle layer. The small reverse current is com-posed of tunneling currentsI tun,n , I tun,p that tunnel throughthe barriers at the middle spacer layer and thermal diffusioncurrentsI th,n , I th,p that flow over these barriers. All thesecurrents depend on the barrier height at the middle spacerlayer. When the reverse bias approaches 1 V, however, thebarriers at the middle spacer layer diminish and the currentsincrease drastically. This causes the breakdownlike increaseof the reverse current at 1 V asshown in Fig. 1.

FIG. 3. Arrhenius plot under different reverse biases. The measurementswere performed using HP4142 in voltage mode.

FIG. 2. Band diagrams for XZ024 under different bias conditions;~a! Zerobias,~b! forward bias~off state!, ~c! forward bias~on state!, and~d! reversebias.

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To confirm our hypothesis, we performed a temperaturedependence study of both the reverse and forward~off state!I –V characteristics. Figure 3 is the Arrhenius plot underdifferent reverse biases. At low temperatures, these curveshave a small temperature dependence, showing a tunnelingnature of the current. At high temperatures, these curves tendto merge, indicating that thermal current predominates. Theslopes of the fourI –V curves near room temperature corre-spond to barrier heights of 0.28, 0.17, 0.05, and 0.01 V,respectively. This is consistent with the value (Vbi2Va),whereVa is the applied voltage.

Shown in Fig. 4 is a semilog plot of the forwardI –Vcharacteristics before switching to the on state. The forwardcurrent increases with temperature while the switch-on volt-age decreases with temperature. This is consistent with inter-band tunneling. TheI –V curves also show some smallwobbles, but the slopes of the curves change little with tem-perature. This is again consistent with interbandtunneling.15–17 Because Si has an indirect band gap, phonon

is needed to preserve the momentum conservation duringtunneling. This leads to the wobbles of theI –V curves.

In summary, we have demonstrated the bistability in a Sidiode containing twodp- i -dn junctions grown by the solidsource MBE. The bistableI –V characteristics are explainedby a band switching theory and a temperature dependencemeasurement further supports the proposed mechanism.Positive feedback processes are also described, which are infavor of high speed operation of the switching device.

This work was supported in part by SRC and NSF.

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FIG. 4. Semilog plot of the forwardI –V characteristics at different tem-peratures up to but right before switching to the on state. The measurementswere performed using a HP4142 in voltage mode with a current complianceof 100 mA. The switch on voltage decreases with increasing temperature.

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