Si/SiGe interband tunneling diodes

R. Duschl, O.G. Schmidt and K. Eberl

Semiconductor tunneling structures have been studied intensively for many years since the pioneering work by Esaki in 1958. The interest arises mainly from possible applications in high-frequency and fast digital devices. Especially in the case of logic circuits the in-troduction of structures with negative differential resistance (NDR) can reduce the circuit complexity and/or increase the speed and reduce the power consumption compared to con-ventional logic circuits. For realistic applications, a peak current density (PCD) of about 10 kA/cm²and a peak to valley current ratio (PVCR) of at least 5 is required.

In principle there are two different types of tunneling structures exhibiting NDR: reso-nant tunneling diodes (RTD) based on double barrier quantum wells and Esaki interband tunneling diodes exploiting the band bending at a p⁺/ n⁺ junction. The performance of state-of-the-art Si/SiGe RTDs, however, is strictly limited by their intrinsic properties. In particular large effective masses and relatively low barrier heights in Si/SiGe heterostruc-tures cause PVCRs which are not much larger than one at room temperature. For this reason, Si(Ge) Esaki diodes are under detailed investigation. First Si interband tunneling diodes produced by an alloy process showed a PCD of about 1 kA/cm² and a maximum PVCR of 3.9 but these values are difficult to reproduce. Due to severe problems in growing high-quality Si/SiGe layers with very high and abrupt doping profiles the PVCR reached in first epitaxially grown structures were limited to about two.

We performed a detailed growth study of Si / SiGe / Si (p⁺/ i / n⁺) interband tunneling diodes. The doping profiles and the structural quality of the highly doped Si layers were carefully optimized and investigated by secondary ion mass spectroscopy, reflec-tion high energy electron diffracreflec-tion and X-ray diffracreflec-tion [ Duschl et al., Electr. Lett. 35, 1111 (1999) ]. The investigated structures consist of a B doped Si p⁺ (510¹⁹ cm ³) buffer layer, followed by a p-type d-doping layer (110¹⁴ cm ²), a 0–1 nm Si / 2–4 nm Si_{1 x}Ge_x/ 1 nm Si intrinsic i-zone, a n-type d-doping layer (110¹⁴ cm ²) and a P doped

needed to avoid dopant segregation. For the measurements, mesa diodes were defined by standard optical lithography and wet chemical etching. The active area of the mesas ranges from 4m²to 4000m².

The tunneling barrier of the structures is determined by the energy gap, which can be tuned by the Ge content, and the width of the intrinsic or depleted region (inset of Fig. 75). At small applied forward voltages, electrons can tunnel from the occupied states in the con-duction band (CB) at the n side to the empty states below the valence band (VB) at the p side. Since molecular beam epitaxy allows the realization of heterostructures with very sharp and well defined profiles the dependence of the relevant values, the PCD and the PVCR, on the different parameters is given quantitatively.

Figure 75: RT I-V characteris-tics and PVCR for diodes with 2, 2.5, 3, 3.4, and 4 nm Si₀:52Ge₀:48

embedded between two 1 nm Si layers within the i-zone.

The schematic band structure is shown in the lower right part.

Fig. 75 shows the RT I-V characteristics as a function of SiGe layer thickness for a Ge con-centration of 48%. The thickness is varied from 2 nm to 4 nm. The shift of the peak voltage with increasing current is explained by a serial resistance within the measurement setup.

A pronounced increase of the current with decreasing width of the i-zone is observed, as would be expected for thinner barriers and hence increased tunneling probabilities. The resulting PCD for the 200 m² mesas is about 0.4 kA/cm² for the structure with 4 nm SiGe and reaches a maximum value of 12.6 kA/cm²for the structure with 2 nm SiGe. The PVCR shows a maximum of 5.1 for the tunneling diode with 3 nm SiGe (Fig. 75). Since a thickness of 3 nm is at the upper limit for the epitaxial growth of Si₀:52Ge₀:48 a further increase of the layer thickness causes an increase of the defect density, which reduces the PVCR. A very thin SiGe layer (2 nm) would avoid these problems but leads to an increase of the leakage current.

For a further optimization, the amount of Ge was varied for the structure with 3 nm SiGe.

The strain in the SiGe layer, however, limits the Ge content for pseudomorphic growth.

A higher Ge content for a fixed layer thickness has the same consequences as an increase

SiGe layer within the i-zone was found to be crucial for the tunneling probability and therefore for the I-V characteristic [ Duschl et al., Physica E (2000), in print ]. Growing the SiGe layer directly at the^ÆP layer without any Si spacer layer deteriorates the device performance. Whereas the growth of the SiGe layer directly at the^ÆB layer reduces the B diffusion and supports the transfer of the holes into the SiGe layer. Both effects increase the tunneling probability. For this reason the 1 nm Si spacer between the ^ÆB and SiGe layers was left out for the optimized structure with 3 nm Si₀:52Ge₀:48. The influence on the j-V characteristic is demonstrated in Fig. 76. The PCD increases from about 3 kA/cm²to more than 8 kA/cm²for the structure without spacer. Even more important is that, despite these high PCD, the PVCR raises from 5.1 to 5.45. This is the highest value ever reported for Si based interband tunneling diodes.

0 2 4 6 8

0.0 0.2 0.4 0.6 0.8 1.0

Si spacer without

Si spacer

j (kA/cm

)

U (V)

Figure 76: RT current density (j)-voltage characteristics of the tunneling structure with 1 nm Si / 3 nm Si₀:52Ge₀:48/ 1 nm Si in the i-zone (with Si spacer), compared to a structure with 3 nm Si₀:52Ge₀:48/ 1 nm Si in the i-zone (without Si spacer).

In conclusion, our study demonstrates the high potential of epitaxially grown Si/SiGe inter-band tunneling diodes. By changing the thickness of the intrinsic SiGe layer the PCD can be varied in a wide range from 0.4 kA/cm²to 12.6 kA/cm². An optimization of the struc-ture regarding Ge content, thickness and position of the SiGe layer results in a diode with a high PCD of 8 kA/cm²and a record PVCR of 5.45 for Si based interband tunneling diodes.

The results imply that the aforementioned minimum requirements for an application in the field of digital circuits can be fulfilled with Si based structures.