Si - Based Tunnel Diode Operation and Forecasted Performance

Transcription

1 Si - Based Tunnel Diode Operation and Forecasted Performance Roger Lake Raytheon Systems Dallas, TX

2 Si / Si x Ge -x Interband Tunnel Diodes The main tunneling process is LA and TO phonon assisted tunneling from the k x and k y electron X- valleys to the hole Γ-valley. The direct tunneling of the k z electrons is negligible since their mass in the tunnel direction is 4 times that of the k xy electrons resulting in twice the decay constant. z Perpendicular Current Flow y x Si wafer k x

3 Si / Si.5 Ge.5 Tunnel Diode Design as of 4/98 Two items that we will change. 8 nm Si.5 Ge.5 Doping throughout Si.5 Ge.5 V=.2 V X xy and X z states X z X xy δ-doped 2 p δ-doped 2 n LH and HH states 8 nm Si.5 Ge.5

4 First working design built 5/98 shown at right. Since then, improvement has been rapid. A peak-to-valley-current-ratio (PVCR) of 4.2 with a current density of 3 ka/cm 2 has been achieved with this exact design substituting P doping for Sb (R. Dushcl et al., Electronics Lett., 35, (999)). Cutting the length of the Si.5 Ge.5 region down from 4nm to 2nm, a current density of 22. ka/cm 2 with a PVCR of 2 was obtained shown below. (S. Rommel et al., 998 IEDM Technical Digest (IEEE, New York, 998) p. 35.) First Working Design V =.325 V T = 3K Si X xy X z X z HH LH X xy Si.5 Ge.5 S. Rommel et al., APL, 73, 29 (998). 2 Β δ-doped HH LH T = 3 K Si/SiGe 6 different devices Si Current (ma) PVCR: 2.5 Current Density: 22. ka/cm Sb δ-doped VOLTAGE (V) ud87.p CURRENT (ma)

5 Figures of Merit - Speed Index High speed switching applications, the Speed Index is the important figure of merit []. Speed Index = I p / C (volts / second) where C is the tunnel diode capacitance and I p is the peak current. Optimization C linearly increases as tunnel junction width is reduced. I p exponentially increases as tunnel junction width is reduced. ==> Minimize tunnel junction width and maximize current I p. C linearly increases with dielectric constant e Si =.9; e Ge = 6.. I p exponentially increases as bandgap decreased, E GSi =.2 ev; E GGe =.66 ev. ==> Maximize I p using Si x Ge -x tunnel region. For a 3.5 nm spacer with 5 A/cm 2 peak current (see figure right), the speed index is calculated to be 3 V GHz, i.e., V can be switched at 3 GHz. Peak Current Density (A/cm 2 ) Si RITD Samples (Series II) 65 o C, minute Anneal Measured Peak Current Densities Theoretical Fit Spacer Thickness (nm) Current density vs. intrinsic-spacer thickness (S. Rommel, PhD thesis, U. Del. 999). [] A. Seabaugh and R. Lake, Tunnel Diodes, in Encyclopedia of Appl. Phys., Vol. 22 (Wiley-VCH, NY, 99) p. 335.

6 Figures of Merit - Peak to Valley Current Ratio (PVCR) Both the peak currents and valley (excess) currents are tunnel currents. They are not thermally activated (see figure (a)). The physical mechanisms governing the valley current are still not understood. 3 2 (a) Current (ma) nm Si.65 Ge.35 nm δ-doping offset µm diameter diodes (w/ bond pads) 7 o C, min anneal 325 o C, min silicidation step.5 K 4.2 K 2 K 45 K 7 K K 2 K 4 K 6 K 8 K 2 K (a) Temperature dependence of tunnel current. S. Rommel, PhD thesis, U. Del. 999.

7 Figures of Merit - Peak to Valley Current Ratio (PVCR) Tunneling from and through band-tails resulting from the heavy doping is one likely mechanism for the excess current []. Optimization Keep dopants out of the tunnel region. A PVCR of 4.2 has already been published and further optimization is highly probable. It is not yet possible to give a realistic maximum theoretical value. Our numerical calculations (see below) show the PVCR increasing from.9 to 3.4 to 7.7 as the bandtails are reduced by a factor of 2 and 2.5. Note that all of the current in figure (b) is tunnel current - not thermally activated p-n diode current. Also, compare with Fig. 4. of [] Sb segregation d-sb d-b e8 B [] Chynoweth, Feldman, and Logan, Phys. Rev., 2, 684 (96). Current (A/cm 2 ) mev 5 mev 2 mev C B Calculated I-Vs with band-tails. 2nd neighbor sp 3 s* band model. Direct, TA, and TO phonon-assisted tunneling. Non-equilibrium Green function approach. V A Excess current mechanism C. Rivas et al., unpublished.

8 The features around.5 -.6V from gap states. Figures of Merit - Peak to Valley Current Ratio (PVCR) Gap States Low T grown heavily doped Si and Si x Ge x need to be understood (or at least empirically characterized). Gap States? Sb segregation d-sb e8 B d-b P. Thompson et al., Appl. Phys. Lett., 75, 38 (999) Current (ma) 7 C 65 C 6 C as grown mev 5 mev

9 Key to Optimization of both Peak and Valley Currents - Precise Control of Dopant Profiles Effect of Sb segregation The Sb segregates towards the surface resulting in Wider tunnel barrier => Lower current density Opportunity for higher peak current densities by confining the Sb. NB: Peak current is exponentially dependent on the tunnel barrier width. Current (ma) PVCR: 2.5 Current Density: 22. ka/cm E F Ideal doping profile. T=3K V=.2 V X xy X z HH X z LH SO X xy HH SO LH Using SIMS doping data T=3K V=.2 V E F Sb δ-doped B δ-doped S. Rommel et al., 998 IEDM Technical Digest (IEEE, New York, 998) p. 35. X z HH LH SO X xy HH SO LH

10 Key to Optimization - Precise Control of Dopant Profiles Effect of B diffusion As dopants diffuse and compensate, the tunnel barrier widens; the speed index and PVCR are reduced. Maximize PVCR by keeping dopants out of the tunnel region. Maximize speed index by minimizing the length of the tunnel region. ==> Together this implies huge doping gradients. Consider diffusion barriers. Compare different dopant species Sb segregation d-sb d-b e8 B.3.2. Current (ma) Concentration (cm -3 ) B as grown B RTA Sb as grown Sb RTA Dopant profiles before and after rapid thermal anneal []. - As Grown RTA 7 -. o C [] P. Thompson et al., Appl. Phys. Lett., 75, 38 (999) Tunnel junction before and after RTA and calculated peak current (inset) [].

11 Open Questions: Dopant Species How do they affect device performance? Sb / B Doping 5 Sb segregates but does not diffuse. Historically, PVCR was highest with Sb doping, second highest with As doping, and lowest with P doping for both Si and Ge tunnel diodes []. Currently, the highest PVCR comes from P doped devices [2]. P / B Doping Current (ma) 7 C 65 C 6 C As grown Current (A/cm 2 ) 4 64 C 68 C 7 C 72 C 75 C 775 C Effect of RTA temperature P. Thompson et al., Appl. Phys. Lett., 75, 38 (999) 8 C Effect of RTA temperature M. Dashiell et al., preprint. [] R. A. Logan and A. G. Chynoweth, Phys. Rev., 3, 89 (963). [2] R. Dushcl et al., Electronics Lett., 35, (999).

12 Is it necessary? How does it affect device performance? The highest PVCR has been observed in d-doped devices []. The highest current density has been observed in non-delta doped device [2]. What is the effect of the quantum states in the d-doped wells? Sb segregation d-sb d-b [] R. Dushcl et al., Electronics Lett., 35, (999). [2] M. Dashiell et al., preprint. Open Questions: Delta Doping e8 B Current (A/cm 2 ) Current (A/cm 2 ) mev 5 mev 2 mev Uncrossing of quantum states? C. Rivas et al., unpublished.

13 Open Questions: The x in the Si x Ge -x Do we need Si x Ge -x? For high current density, minimize x to minimize E G. BUT, the highest current density has been observed in an all-si device []. What is the optimum value of x? [] M. Dashiell et al., preprint.

14 How About Carbon? SiGeC? Diffusion barrier? Offsets? Band Gaps?

15 Forward Looking Issue - Scaling? How small in diameter can we make tunnel diodes? Surface recombination will reduce PVCR. How small do we want to make them? For maximum speed - Minimum area (minimum capacitance) maximum current density diodes.

16 Conclusions First working design built 5/98 followed by rapid progress. Published PVCR of 4.2. Published current densities of 22 ka/cm 2. Speed index - maximize current density ==> minimize intrinsic layer length. Maximum estimated of 3 V GHz. PVCR - maintain clean intrinsic tunnel region. Maximum of 4.2 published with better results already obtained. Nanometer control of dopant profiles is key to optimizing both speed index and PVCR. Processing, dopant species, diffusion barriers. Open Questions: The x in Si x Ge -x? Dopant species? d-doping? SiGeC? Scaling?