Tunable hot-carrier photodetection beyond the bandgap spectral limit

Journal name:
Nature Photonics
Year published:
Published online


The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit λc that is related to the activation energy (or bandgap) of the semiconductor structure (or material) (Δ) through the relationship λc = hc/Δ. This spectral rule dominates device design and intrinsically limits the long-wavelength response of a semiconductor photodetector. Here, we report a new, long-wavelength photodetection principle based on a hot–cold hole energy transfer mechanism that overcomes this spectral limit. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. This enables a very long-wavelength infrared response. In our experiments, we observe a response up to 55 µm, which is tunable by varying the degree of hot-hole injection, for a GaAs/AlGaAs sample with Δ = 0.32 eV (equivalent to 3.9 µm in wavelength).

At a glance


  1. Sample structure and the VLWIR response at 5.3 K.
    Figure 1: Sample structure and the VLWIR response at 5.3 K.

    a, Schematic of the p-type GaAs/AlxGa1–xAs structures. b, Calculated equilibrium valence-band alignment, with and without image-force barrier lowering15 (thick grey and dashed blue lines, respectively). c, Schematic valence-band diagrams (including band bending) under negative bias (positive polarity applied on the injector), with a comparison of hole photoexcitation and emission without (top) and with (bottom) hot–cold hole energy transfer. d, Photoresponse at 5.3 K. The dashed line is the escape-cone model fit. Marked features are associated with GaAs- and AlAs-like phonons. e, Comparison of the response for samples SP1005–1007 and LH1002 at 5.3 K. The optical power spectrum of the FTIR spectrometer used in the experiment (incident on a sample with active area of 260 × 260 µm2) is also shown.

  2. Spectral weight (SW) of response and differential photocurrents.
    Figure 2: Spectral weight (SW) of response and differential photocurrents.

    a, Variation of SW (sample SP1007) with bias and λmin, calculated using equation (1) with λmax = 55 µm. b, Bias-dependent SW. Values used for λmax determine SWtot and SWpump. SWtot corresponds to all of the holes being collected. SWpump is calculated using λmax = 2.95 µm (that is, 0.42 eV, the maximum of the graded barrier). SWtot displays two maxima compared to SWpump, due to the bias-dependent VLWIR response. c, Differential SWpump (proportional to ) (top) and differential photocurrents measured using different optical excitation sources (bottom).

  3. Photoresponse obtained using long-pass filters and an external optical excitation source.
    Figure 3: Photoresponse obtained using long-pass filters and an external optical excitation source.

    a, Experimental apparatus, where the semi-insulating GaAs is double-side-polished and acts as a beam splitter. bd, SW of the response (sample SP1007) measured using long-pass filters with cut-on wavelengths (λCO) of 2.4 µm (b), 3.6 µm (c) and 4.5 µm (d), respectively. The VLWIR response is graduallly diminished by increasing λCO, and is barely seen when a λCO = 4.5 µm filter is used. e, Recovery of the VLWIR response by providing hot holes through an external optical excitation source, measured with a 4.5 µm long-pass filter.

  4. Tuning the VLWIR response.
    Figure 4: Tuning the VLWIR response.

    a, SW of response at different excitation levels by controlling the power of an optical source (see Fig. 3a for experimental apparatus). The experiment was carried out on sample SP1006, which has a very similar VLWIR response to SP1007. b, Power spectra of the optical source (incident on the sample with an active area of 260 × 260 µm2). A quartz glass filter is used to block the long-wavelength portion (up to 4.8 µm). c,d, Dependence of the VLWIR response (at −0.1 V) on excitation power. Indicated by arrows are the cut-on wavelength of the filter and the 2 × TO(X) phonon feature of the GaAs beam splitter31. e, Comparison of the upconverter33 and proposed hot-carrier photodetector.


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Author information


  1. Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA

    • Yan-Feng Lao &
    • A. G. Unil Perera
  2. School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK

    • L. H. Li,
    • S. P. Khanna &
    • E. H. Linfield
  3. Key Laboratory of Artificial Structures and Quantum Control, Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China

    • H. C. Liu


A.G.U.P. conceived the split-off heterojunction concept and was involved in designing the device structure. Y.F.L. and A.G.U.P. conceived the experiments and wrote the paper. L.H.L., S.P.K. and E.H.L. grew the samples using molecular beam epitaxy. H.C.L. carried out the device processing. Y.F.L. performed electrical and optical measurements and data analysis. A.G.U.P. guided the project. All authors contributed to the content in the paper.

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