W. Z. Shen

NEGATIVE CAPACITANCE OF GAAS HOMOJUNCTION FAR-INFRARED DETECTORS

Bias, frequency and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. Unlike in other devices, even up to 1 MHz in HIWIP, the negative capacitance value keeps increasing with frequency, giving a stronger effect. The origin of this effect is believed to be due to the carrier capture and emission at interface states. Fitting data based on charging-discharging current and the inertial conducting current model show good agreement with the experimental observations.

Bias effects in high performance GaAs homojunction far-infrared detectors

A high performance, bias tunable, p-GaAs homojunction interfacial workfunction internal photoemission far-infrared detector is demonstrated. A responsivity of 3.10±0.05 A/W, a quantum efficiency of 12.5%, and a detectivity D* of 5.9×1010 cmHz/W were obtained at 4.2 K for cutoff wavelengths from 80 to 100 μm. The bias dependences of the quantum efficiency, detectivity, and cutoff wavelength were measured and are well explained by the theoretical model. The effect of the layer number on detector performance and the uniformity of the detectors are discussed. A comparison with Ge:Ga photoconductive detectors suggests that similar or even better performance may be obtainable with a far-infrared detector.

GaAs/AlGaAs quantum well photodetectors with a cutoff wavelength at 28 μm

We demonstrate the longest (λc=28.6 μm) far-infrared quantum well photodetectors (QWIPs) based on a bound-to-bound intersubband transition in GaAs/AlGaAs. The responsivity is comparable with that of mid-infrared GaAs/AlGaAs and InGaAs/GaAs QWIPs. A peak responsivity of 0.265 A/W and detectivity of 2.5×109 cmHz/W at a wavelength of 26.9 μm and 4.2 K have been achieved. Based on the temperature dependent dark current and responsivity results, it is expected that similar performance can be obtained at least up to 20 K.

GaAs multilayer p+-i homojunction far-infrared detectors

A molecular beam epitaxy grown wavelength tunable GaAs p+-i homojunction interfacial work-function internal photoemission far-infrared detector is developed. The multilayer (p+-i-p+-i-…) detector structures consist of 2, 5, and 10 emitter layers. Experimental results are explained in terms of the number of emitter layers and the doping concentrations of the emitter layer. A detector with 10 multilayers and an emitter layer doping concentration (Ne) of 3×1018 cm−3 shows a current responsivity of 2 A/W, an effective quantum efficiency of 9.2% (at 26.3 μm) with a cutoff wavelength of 85 μ and the noise equivalent power of 2.18×10−12 W/Hz at 4.2 K.

Demonstration of Si homojunction far-infrared detectors

A 48 μm cutoff wavelength (λc) Si far-infrared (FIR) detector is demonstrated. Internal photoemission over a Si interfacial work-function of a homojunction consisting of molecular beam epitaxy grown multilayers (p+ emitter layers and intrinsic layers) is employed. The detector shows high responsivity over a wide wavelength range with a peak responsivity of 12.3±0.1 A/W at 27.5 μm and detectivity D* of 6.6×1010 cmHz/W. The λc and bias dependent quantum efficiency agree well with theory. Based on the experimental results and the model, Si FIR detectors (40–200 μm) with high performance and tailorable λcs can be realized using higher emitter layer doping concentrations.

Free-carrier absorption in Be- and C-doped GaAs epilayers and far infrared detector applications

Far infrared (FIR) absorption, reflection, and transmission in heavily doped p-GaAs multilayer structures have been measured for wavelengths 20–200 μm and compared with the calculated results. Both Be (in the range 3×1018–2.6×1019 cm−3) and C (1.8×1018–4.7×1019 cm−3)-doped structures were studied. It is found that the observed absorption, reflection, and transmission are explained correctly by the model with a dominant role of free-carrier absorption in highly doped regions. High reflection from heavily doped thick layers is attractive for the resonant cavity enhanced FIR detectors.

Absorption spectroscopy studies of strained InGaAs/GaAs single‐quantum wells

Strained In0.20Ga0.80As/GaAs single‐quantum well (SQW) structures with the GaAs capping layer thickness ranging from 5 to 500 nm have been studied directly by absorption spectroscopy. The absorption peaks are shifted to lower energy while the GaAs capping layer thickness decreases due to the strain relaxation in InGaAs/GaAs SQW structures induced by the GaAs capping layer. The best fit gives the conduction‐band offset ratio Qc=0.70±0.05. The pronounced exciton peaks are observed in the absorption spectra at room temperature. The strength of the exciton–phonon coupling is determined from linewidth analysis and is found to be much larger than that of strained InGaAs/GaAs MQW structures.

Effect of emitter layer concentration on the performance of GaAs p/sup +/-i homojunction far-infrared detectors: a comparison of theory and experiment

The performance of GaAs multilayer (p/sup +/-i-p/sup +/-i-...) homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors as a function of emitter layer (p/sup +/) concentration is reported. The dark current characteristics have been investigated and compared with a model which includes the space charge, tunneling, and multiple-image-force effects. The experimentally determined detector cutoff wavelength is found to be in reasonable agreement with the high density (HD) theory. The detector responsivity follows well the quantum efficiency predicted by scaling the free carrier absorption coefficient linearly with the doping concentration. All these comparisons are necessary to design and optimize GaAs HIWIP FIR detectors.

Far-infrared free-hole absorption in epitaxial silicon films for homojunction detectors

We report on the investigation of free-carrier absorption characteristics for epitaxially grown p-type silicon thin films in the far-infrared region (50–200 μm), where Si homojunction interfacial workfunction internal photoemission (HIWIP) detectors are employed. Five Si thin films were grown by molecular beam epitaxy on different silicon substrates over a range of carrier concentrations, and the experimental absorption data were compared with calculated results. The free-hole absorption is found to be almost independent of the measured wavelength. A linear regression relationship between the absorption coefficient and the carrier concentration, in agreement with theory, has been obtained and employed to calculate the photon absorption probability in HIWIP detectors.

A spectroscopic study of GaAs homojunction internal photoemission far infrared detectors

Abstract We report a spectroscopic study of absorption and photoconductivity in GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors utilizing molecular beam epitaxy (MBE) grown multilayer (p+−p−−p+−p−) structures. Strong FIR (50–200 μm) free carrier absorption has been observed and analyzed for a p+ GaAs thin film, revealing the suitability for FIR detection. The basic physical mechanism of free carrier absorption in the HIWIP FIR detectors has been determined to be an acoustic phonon-emission assisted process. A simple recombination model is proposed to account for the bias dependence of the responsivity and the saturation behavior. Using the measured responsivity and dark current data, detectivity (D λ ∗ ) of the FIR detectors has also been estimated.