Relatos alternos y emergentes.

There are three main mechanisms limiting the response time of HEIWIP detectors. These are the intrinsic relaxation time, the carrier transit time, and the RC time constant. The intrinsic time response of the p-GaAs/AlGaAs HEIWIP detector can be estimated from the bias dependent responsivity measurements 31. Under irradiation, excited carriers

Figure 2.24: The solid line shows the model detectivity spectra for the 2409 HEIWIP detector in the BLIP regime (λ0 is 70μm). Background temperature is TBG = 300 K and FOV = 180◦. The BLIP temperature, T_BLIP = 13 K. Model detectivities for structures similar to 2409, but with a 1.5μm-thick n-type buffer layer doped to 1×1019_cm−3_{, or with} an-type bottom contact, or with the emitter doping density increased to 1×1019 cm−3 are shown for comparison. The top solid line represents the detectivity of an ideal detector with the sameλ₀.

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Figure 2.25: Bias dependence of responsivity in 2409 HEIWIP FIR detector with 30 emit- ter/barrier units; λ₀ = 70 μm.

are generated by free carrier absorption in the valance band. However, the total number of carriers remains constant with an effective temperature Teff (which deviates from the equilibrium temperature, T₀) in a hot carrier population. The photoconductivity is given by:

Δσ =qp[μ(T_eff)−μ(T₀)] (2.38) where p is the carrier density, μ is the hole mobility. The change in conductivity under illumination leads to a change in the current in the external circuit (photocurrent) ofI_photo= ΔσF A, whereF is the applied field in the detector and Ais its area. A minute heating of the carriers leads to:

Δσ=qpdμ dT T=T0 ·(T_eff−T0) (2.39)

Therefore, the photoconductivity is directly related to the transport properties of the hot carriers. The temperature riseT_eff−T0 has now to be related to the incident powerP from the energy balance equation :

pk(T_eff −T₀)

τ = P η

Ad (2.40)

wherekis the Boltzmann’s constant, dis the detector thickness,τ is the energy relaxation time of the hot carriers, which in the limit, can be regarded as the detector response time. The left side term of Eq. 2.40 represents the power transferred to the lattice by hot carriers, and the right side term displays the power transferred to the hot carrier distribution. The current responsivityR=I_photo/P under a bias voltage V can be expressed as:

R= qητ kd2 dμ dT T=T0 ·V (2.41)

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Using the temperature dependent mobilityμ∝T3/2, due to the ionized impurity scattering for the low experimental temperature ofT0 = 4.2 K, the current responsivity can be rewritten as:

R= 3ημτ 2d2

q

kT₀V (2.42)

The bias dependence of responsivity measured at 4.2 K in a p-GaAs HEIWIP FIR detector with 30 periods and λ0 around 70 μm is shown in Fig. 2.25. The measured responsivity increases linearly with the bias at low bias voltages as predicted by Eq. 2.42. The saturation of the responsivity at high bias is due to the quasi-depletion of the impurity band as proposed in a simple recombination model32. For a given bias, the responsivity is proportional to the response time, which is the same as in the case of an intrinsic or extrinsic photoconductive detector. The detector 2409 has a total absorption quantum efficiency of 0.07 at 14 μm, thickness of 3 μm, and a slope of 0.78 A/WV in the linear region of the

R vs.V graph in Fig. 2.25. Using the above parameters the response time of this detector, determined by the energy relaxation time, is estimated to be about τ_relax ∼= 6×10−12 s. Assuming that the hole mobility to be μ = 60 cm2/Vs, the transit time of the carrier through this structure, at an electric field of 3 kV/cm, was estimated to beτ_trans∼= 10−9 s. In practice, the most serious limitation arises from theτRC = RtotalC constant, whereC is the capacitance of the detector andRtotal=RLRd/(RL+Rd) is the equivalent resistance, where R_L is the load andR_d is the dynamic resistance of the detector. Typical values of R_d for a HEIWIP detector with a threshold wavelength λ₀ = 70 μm are about 5×105 _{and 5×10}4 _{Ω for a bias of 0.5 and 0.7 V, respectively.}

is about 6 pF. For a very high load resistance (RLRd) theRCtime constant is estimated to be 3×10−7 s. Increasing the detector size up to 800 μm×800 μm will increase the time constant up to 1.2×10−6 s. An estimation shows that the intrinsic time response of HEIWIP detectors is very fast, and in real situations, the detector time response is mainly restricted by the RC time constant of it.

Chapter 3

Homojunction Interfacial

Workfunction Far Infrared

Detectors

3.1

Introduction

The basic structure of a HIWIP detector28 consists of a heavily doped emitter layer and a barrier layer sandwiched between the contact layers. Forp-type structures, the inter- facial workfunction, Δ, is the offset between the Fermi level of the emitter and the valance band edge of the barrier, arising due to band gap narrowing of the highly doped emitter layer. The detection mechanism involves free carrier absorption in the emitter layer, followed by the internal photoemission of photoexcited carriers across the interfacial barrier. These photoemitted carriers are swept out of the active region by the applied electric field and are

collected at the contact. Initially, it was believed that, in principle, GaAs FIR detectors could be designed with arbitrarily long threshold wavelength [λ0 = 1240 (meV μm)/Δ]33 because Δ can be made arbitrarily small by increasing the doping density of the emitters34. For example, it has been shown35 that λ0 was tunable from 76 to 85 μm by varying the Be:dopant density of the emitter layer from 1×1018 to 3×1018 cm−3. However, it was found that the optically induced transition of carriers from the heavy-hole (HH) to the light-hole (LH) band, in highly doped p-type emitters depletes states close to Fermi level of the emitter, thereby increasing the effective workfunction for photoemission36. In other words, λ₀ of HIWIP detectors is limited within 100 μm. Therefore, it is important that

p-type HIWIP detectors are designed to operate below the threshold limit set by the on- set of free carrier state depletion through transitions from HH-hole to LH-hole band. The experimental results observed for three Carbon dopedp-type GaAs HIWIP FIR detectors with different barrier thicknesses are presented in this chapter.