Si doped GaAs/AlGaAs terahertz detector and phonon effect on the responsivity

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Abstract

Terahertz detection capability of an n-type heterojunction interfacial work function internal photoemission (HEIWIP) detector is demonstrated. Threshold frequency, f0, of 3.2 THz (93 μm) was obtained by using n-type GaAs emitter doped to 1 × 1018 cm−3 and Al0.04Ga0.96As single barrier structure. The detector shows a broad spectral response from 30 to 3.2 THz (10–93 μm) with peak responsivity of 6.5 A/W at 7.1 THz under a forward bias field of 0.7 kV/cm at 6 K. The peak quantum efficiency and peak detectivity are ∼19% and ∼5.5 × 108 Jones, respectively under a bias field of 0.7 kV/cm at 6 K. In addition, the detector can be operated up to 25 K.

Introduction

In recent years, terahertz detectors (0.1–30 THz) have been the center of attraction in many areas such as medical diagnostic, security, astronomy, communication, etc. Numerous advantages can be achieved upon the availability of a well developed terahertz detector. Bolometers and pyroelectric detectors are currently the most popular detectors in the THz region. However, a main drawback of these detectors is the slow photoresponse which hinders development of many promising THz applications. In addition, the difficulty of integrating these detectors into focal plane array for terahertz imaging. Therefore, photon detectors which possess faster photoresponse and focal plane array capability, are good candidates for THz applications.

In HEIWIP detectors, an undoped alloy semiconductor material is used as the barrier and highly doped semiconductor as the emitter. The internal work function, Δ, is defined from the top of the Fermi energy in the emitter to the bottom of the conduction band of the barrier. The Internal work function is given by Δ = Δx + Δd, and Δd = Δnarr − EF, where Δx is the conduction band offset between the emitter and the barrier due to composition, Δnarr is the band gap narrowing in the emitter layer due to doping, and EF is the Fermi energy. The zero response threshold f0 is determined by the energy difference from the Fermi level in the emitter to the bottom of the conduction band of the barrier. The threshold frequency of the detector, f0, can be tailored by changing the alloy fraction, x[1], [2]. Threshold frequency, f0, is given by f0 = Δ/4.133 in terahertz. Here Δ is in meV.

The reported results on HEIWIP detectors are limited to p-type structures [3], [4]. p-type HEIWIP detectors have shown the ability to push the threshold limit beyond 5 THz (>60 μm). Tailorability of threshold frequency f0 with different Al fractions in p-type HEIWIP terahertz detectors has been shown for three detectors with f0 = 4.6, 3.6, and 3.2 THz [2]. The Al fraction used for the 3.2 THz threshold detector is 0.005. This 0.005 Al fraction is close to the practical lowest limit for MBE growth. Therefore, lowering the work function further may not be possible. One alternative to this is to use an inverted HEIWIP structure [4]. In the inverted structure, p-doped AlxGa1−xAs is used as the emitters and undoped GaAs is used as the barriers. Therefore, smaller work function can be designed by increasing the Al fraction and thereby a lower threshold frequency can be achieved. However, the reported lowest threshold obtained with inverted structure is 2.3 THz (128 μm). The other alternative is to use an n-type HEIWIP detectors. For the same small Al fraction composition, the work function in n-type HEIWIP is smaller than the that of p-type HEIWIP because of the smaller effective mass of electrons compared to the effective mass of holes. According to initial theoretical calculations, it is possible to achieve the threshold frequency below 3 THz in n-type HEIWIP with relatively larger Al fraction. For an example, theoretically, 1 THz threshold frequency can be obtained in n-type HEIWIP with Al fraction of 0.03. In this paper, the capability of extending the zero response threshold to 3 THz limit by using n-type single barrier HEIWIP is demonstrated.

Section snippets

Device design

The device structure as shown in Fig. 1(a) consists of an undoped 1 μm thick AlxGa1−xAs (x = 0.04) barrier layer sandwiched between two n-doped (Si) 1 × 1018 cm−3 GaAs contact layers with the top contact being 100 nm and the bottom contact being 700 nm in thickness. GaAs doped to 5 × 1018 cm−3 with Si was used as the substrate. The top and the bottom contact layers work as emitters for reverse and forward bias operations. The Si doping concentration and the Al fraction were verified by secondary ion mass

Device characterization

Low temperature dark current–voltage measurements were performed on the device from 4.2 to 120 K. Fig. 2 shows dark current density–voltage behavior for temperatures ranging from 10 to 40 K. The dark current increases rapidly after 1.5 kV/cm in forward bias and −1.0 kV/cm in the reverse bias. Therefore, the device performance is restricted in this bias field region. Spectral measurements were performed by using a fast Fourier transform infrared spectrometer (FTIR). A silicon composite bolometer was

Forward and reverse bias photoresponses

The device photoresponses in the forward and the reverse bias operations are shown in Fig. 4(a). When the device is in forward bias, the photoresponse is from the bottom contact layer, whereas, the photoresponse is from the top contact layer when the device is in reverse bias. The responsivity from the bottom contact layer is much higher than that of the top contact layer. The peak responsivity of the top contact layer is ∼1.2 A/W while it is 6.5 A/W for the bottom contact layer. The reason for

Conclusion

Extending the threshold frequency below 3 THz in photon detectors is possible. Threshold extending below 3 THz has been achieved in HEIWIP whereas it is inherently difficult in QWIP and Schottky barrier photo detectors. The lowest threshold limit obtained in HEIWIP (not for the inverted structure) is 3.2 THz [2] and the Al fraction in the barrier region is 0.005. This 0.005 Al fraction is close to the lowest practical limit that can be achieved for any present growth technology. Therefore, this

Acknowledgement

This work was supported in part by the US NSF under grant No. ECS-0553051.

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