Heterostructure Band Engineering of Type-II InAs/GaSb Superlattice ...

More documents

Recommendations

Info

Spectral Response (a.u.) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 λc=10.8 μm PbIbN PIN λc=11.0 μm 0.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Wavelength (μm) Figure 3. Spectral response of PIN and PbIbN devices at 77K with cutoff wavelength of 11.0 µm and 10.8 µm respectively. Current-voltage (IV) characteristics were measured for all the devices from 30K to 250K using HP4145 semiconductor parameter analyzer. Figure 4(a) shows temperature dependent dark current characteristics for PbIbN detector and figure 4(b) compares it with PIN detector at 77K. At lower temperatures, these devices show asymmetric diode characteristic while at higher temperatures the I-V curves become symmetric owing to the dark current due to thermal generation of carriers. J (A/cm 2 ) 100 10 1 0.1 0.01 1E-3 1E-4 1E-5 -3 -2 -1 0 1 2 3 Bias (V) (a) 77K 150K 200K 250K J (A/cm 2 ) 100 10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 77K -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Bias (V) (b) PIN PbIbN Figure 4. (a) Variable temperature dark current characteristics of PbIbN detector, (b) dark current density comparison of PbIbN design with PIN design at 77K. As shown in figure 4(b), PbIbN design shows significantly improved performance over PIN detector. The major sources of dark current in a PIN detector are thermal generation of carriers, SRH current, tunneling currents and minority carrier diffusion current. In PbIbN design, an electron barrier of wider bandgap material is placed between P contact layer and absorber region I. When the detector is operated under reverse bias, photogenerated holes move towards P contact while electrons move towards N contact. The eB layer allows unimpeded flow of holes while blocking electrons and also leads Proc. of SPIE Vol. 7660 76601T-4 Downloaded from SPIE Digital Library on 21 May 2010 to 64.106.37.191. Terms of Use: http://spiedl.org/terms
to majority of electric field drop across the eB layer and reduced field drop across the active region as compared to PIN diode. Due to significant reduction in field drop in active region, smaller bandgap material, tunneling currents are reduced and also activity of SRH centers in the active region is reduced leading to reduction in dark current. Further, eB layer blocks the minority carrier diffusion from P contact layer into the active region and hence reduces dark current. Similarly, hB layer offers barrier for holes while it allows unimpeded flow of electrons when the device is operated in reverse bias, similar to the role played by eB for holes. Field drop across the active region is further reduced as there are barriers on either side, made of wider bandgap material, which reduce SRH and tunneling dark currents as mentioned above. Also, hB layer reduces dark current further by blocking minority carrier diffusion from N contact layer into the active region. The PbIbN design has all the layers made of SLS, which is a clear demonstration of bandgap as well as band offset tunability in this system by merely changing the thicknesses of InAs and GaSb layers. PbIbN design shows dark current reduction, over PIN design, by more than two orders of magnitude. At -0.5V, PbIbN reduces it by factor of 36 over PIN design. However, this reduction is much more significant at lower bias values, for example, at -50 mV PbIbN shows more than three orders of magnitude reduction in dark current. Radiometric measurements were carried out at 77K and as mentioned earlier, longwave bandpass filter was used for all the measurements. It is to be noted that photocurrent due to 300K background under 2π field of view (FOV) was at least a factor of four higher than the dark current implying background limited (BLIP) operation 9 at 77K for all the three devices. The specific detectivity (D*) was measured for 2π FOV under BLIP condition, and was scaled to f/2 optics. Figure 5(a) shows measured D* (f/2 FOV), not calculated from shot noise, and 5(b) shows 77K responsivity and external quantum efficiency (QE) for PbIbN device at 10 µm wavelength. 77K 4 77K 40 10 9 10 8 D*(cm-Hz 1/2 /W) 1010 Measured D* 2π FOV Measured D* F/2 FOV Responsivity (A/W) 3 2 1 30 20 10 QE (%) at 10 μm . -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Bias (V) (a) 0 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Bias (V) (b) Figure 5. (a) Plot of measured specific detectivity, (b) responsivity and quantum efficiency of PbIbN detector at 77K. As shown in figure 5(a), peak D* for PbIbN design is 2×10 10 (cm-√Hz)/W at -0.5 V. The measured value of responsivity at this bias is 3.4 A/W and QE of 35% is attained at 10 µm wavelength. It is to be noted that the calculated value of shotnoise limited D* is 5.2×10 10 (cm-√Hz)/W for 2π FOV. 4. CONCLUSIONS It has been shown that heterojunction type-II SLS based LWIR photodetectors with unipolar current blocking layers show improved performance over conventional PIN devices. The reported barrier engineered device with two unipolar barrier layers, contact layers made of SLS showed much lower dark current and better signal to noise ratio due to optimal Proc. of SPIE Vol. 7660 76601T-5 Downloaded from SPIE Digital Library on 21 May 2010 to 64.106.37.191. Terms of Use: http://spiedl.org/terms
Page 1 and 2: Invited Paper Heterostructure Band
Page 3: 138nm 13ML/8ML (p = 2.8×10 18 cm -

Heterostructure Band Engineering of Type-II InAs/GaSb Superlattice ...

Create successful ePaper yourself

Delete template?

Save as template?