Yusuke Matsukura

Photodetection around 10 μm wavelength using s-p transitions in InAs/AlAs/AlGaAs self-assembled quantum dots

We propose a quantum dot infrared photodetector (QDIP) having distinct sensitivity to mutually orthogonal in-plane polarized infrared radiation, and applicable to practical infrared (IR) imaging applications. Our QDIP has either an InAs/AlAs/AlGaAs or an AlAs/InAs/AlAs/AlGaAs structure in which extra-thin AlAs layers were introduced underneath the AlGaAs buffer layer to strongly confine the carriers and also to enhance the dot density before the Stranski–Krastanov mode growth of InAs quantum dots was carried out. At 80 K, the detector has high sensitivity to in-plane orthogonally polarized infrared light, and photocurrent responsivity peaks of up to 230 mA/W associated with distinct mutually orthogonal polar lights which were observed at a wavelength of around 10.0 μm.

CF4/O2 PLASMA INDUCED DAMAGE IN SI-IMPLANTED N-TYPE GAAS CRYSTALS

The CF4/O2 or CF4 plasma induced damage in Si-implanted n-type GaAs crystals has been investigated by using Hall measurement and secondary ion mass spectrometry (SIMS) analysis. We found that sheet resistance degraded after plasma exposure and postannealing. The degradation of the sheet resistance during postannealing at 200–400 °C, is mainly associated with the reduction of carrier density in the crystal. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4/O2 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine in the Si-implanted n-type GaAs layer, were observed after postannealing at 400 °C. Based on these findings, a fluorine passivation related model is proposed, where Si–F bonds are formed in the Si-implanted n-type GaAs crystals.

Donor Neutralization by Fluorine Containing Plasmas in Si-Doped n-Type GaAs Crystals

CF4, SF6 and CF4/O2 plasma-induced damage in silicon-doped n-type GaAs crystals have been investigated by using Hall measurements, X-ray Photoelectron Spectroscopy (XPS) analysis and Secondary Ion Mass Spectrometry (SIMS) analysis. We observed that the carrier density degraded after plasma exposure and post annealing. The degree of deactivation of carrier density with post annealing was strongly dependent on the plasma chemistries and the self-bias Vdc of the plasmas. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine atoms in n-type GaAs layers were observed after post annealing at 400°C. From these results, we proposed a mechanism of plasma induced damage in which internally diffused fluorine atoms neutralize the donor silicon by forming a Si–F bond. We also evaluated the effective diffusion coefficient of fluorine in GaAs as 1.5×10-11 cm2/s (at 400°C).

Thermal instability of GaSb surface oxide

In the development of InAs/GaSb Type-II superlattice (T2SL) infrared photodetectors, the surface leakage current at the mesa sidewall must be suppressed. To achieve this requirement, both the surface treatment and the passivation layer are key technologies. As a starting point to design these processes, we investigated the GaSb oxide in terms of its growth and thermal stability. We found that the formation of GaSb oxide was very different from those of GaAs. Both Ga and Sb are oxidized at the surface of GaSb. In contrast, only Ga is oxidized and As is barely oxidized in the case of GaAs. Interestingly, the GaSb oxide can be formed even in DI water, which results in a very thick oxide film over 40 nm after 120 minutes. To examine the thermal stability, the GaSb native oxide was annealed in a vacuum and analyzed by XPS and Raman spectroscopy. These analyses suggest that SbOx in the GaSb native oxide will be reduced to metallic Sb above 300°C. To directly evaluate the effect of oxide instability on the device performance, a T2SL p-i-n photodetector was fabricated that has a cutoff wavelength of about 4 μm at 80 K. As a result, the surface leakage component was increased by the post annealing at 325°C. On the basis of these results, it is possible to speculate that a part of GaSb oxide on the sidewall surface will be reduced to metallic Sb, which acts as an origin of additional leakage current path.

Development of mid-wavelength QWIP FPA

A novel quantum-well infrared photodetector (QWIP) with peak responsivity in the mid-wavelength (MW) range was characterized, and the performance of a focal-plane array (FPA) based on the MW-QWIP was investigated. InGaAs/AlGaAs quantum wells were used for the QWIP, resulting in a peak absorption wavelength in the range of 4.5~5.0 μm. The doping concentration and In composition of the well layers were varied to improve the photocurrent of the QWIP. The relationship between the noise of the QWIP and the number of multi quantum well (MQW) layers was also investigated, and the optical gain g was estimated. The noise-equivalent temperature differences (NETDs) of QWIPs with various numbers of MQW layers were calculated, and the optimum number of MQW layers was evaluated. It was found that controlling the In composition of the wells was very effective for improving the photocurrent. As a result, a MW-QWIP FPA with a NETD of 21 mK at an operation temperature of 80 K, an integration time of 16 ms, and F2.0 optics was fabricated.

Characterization of photocurrent of mid-wavelength quantum-well infrared photodetector (MW-QWIP)

We investigated the mechanism of the photocurrent transmission in mid-wavelength quantum-well infrared photodetectors that were made using InGaAs/AlGaAs quantum wells so that their peak absorption would be at a wavelength near 5 μm. Analyzing the bias-voltage dependence of the photocurrent for the samples with different well layer thicknesses, we found that the photocurrent transmission could be accounted for by taking into account the tunneling process via the triangular barrier, the effect of the intrinsic electric field due to the unintentional impurities, and the effect of the drift velocity.

Effect of diffusion-barrier insertion into the reflector electrode on QWIP device characteristics

We investigated GaAs/AlGaAs QWIP devices that had TiW/Au reflector electrodes and found that the thermal stability, and as a result, the uniformity of the IV characteristics were dramatically improved by the insertion of a TiW diffusion barrier. A secondary ion mass spectroscopy analysis showed that with the TiW insertion, the Au in-diffusion during the thermal process at around 400°C was completely suppressed. A reflectivity measurement of the GaAs/Au and GaAs/TiW/Au structures revealed that the reflectivities were almost the same, indicating the TiW/Au structure was a good candidate for use as a reflector electrode. A comparison of the fabricated GaAs/AlGaAs QWIP devices that had conventional Au with our new TiW/Au reflector electrodes showed that our devices exhibits a performance comparable with that of the conventional devices because of their similar reflectivity. Furthermore, the uniformity of the IV characteristics of more than 50 devices was greatly improved, especially in the reverse-biased region where the upper AlGaAs barrier (nearest to the reflector) served as an electron-emitter.

Characterization of the dark current of a quantum well infrared photodetector (QWIP) with selectively doped barrier layers

We investigated the behavior of the dark current (Id) in quantum well infrared photodetectors (QWIPs) in which the barrier layers were selectively doped instead of the well layers. Because the selective doping bends the conduction band (CB) edge in the portion of the barrier near the interface, the mechanism by which carriers in the wells can be emitted over the barriers, i.e. thermal emission and tunneling through this portion of the barrier, could be emphasized. We first confirmed that selectively doping the barrier layers clearly affects the Id-V characteristics. Then, by evaluating the activation energy obtained from the temperature dependence of Id, we found that the Poole-Frenkel emission (PFE) mechanism and the thermal-assisted tunneling (TAT)-like mechanism are dominant in the lower bias and higher bias regions, respectively.

Absorption spectrum observation of quantum well infrared photodetectors by new prism attached attenuated total reflection

We developed a very simple and useful method for observing the optical absorption due to intersubband transition in quantum wells. This new technique based on attenuated total reflection (ATR) is applied to evaluate epitaxial wafers containing multi quantum wells (MQW), used for quantum well infrared photodetectors (QWIP). For the intersubband transition, normal incidence produces only weak absorption because of the quantum selection rules on polarization. ATR has been used to emphasize the intersubband absorption, however, the GaAs substrate including MQW had to be made into a prism. We observed intersubband absorption without such preparation of GaAs substrate by just attaching a Ge prism to it. Attached prism in ATR is normally used for collecting the absorption near the sample surface by utilizing the total reflection at the prism-sample interface. However, we adjusted the angle of incidence to propagate the refracted light into the sample because absorption occurred inside epitaxial layers in the QWIP structure. Using this modified prism attached ATR, we were able to measure the intersubband absorption more clearly than in the case of conventional prism attached ATR. Due to avoid the complicated sample preparation, we could easily compare the absorption of MQW with QWIP device characteristics.

Sensitivity enhancement of quantum well infrared photodetectors with a pseudorandom optical coupler by confinement of internal multireflection

We investigated the sensitivity enhancement of quantum well IR photodetectors (QWIP) with the pseudo-random optical coupler. We fabricated QWIP focal plane arrays (FPA) containing AlGaAs/GaAs multi-quantum wells (MQW) detecting 7-8 micrometers IR regions. After QWIP-FPA and read-out integrated circuits were hybridized, GaAs substrates attached to the back of the FPA were removed. We compared the sensitivity of a QWIP-FPA without a GaAs substrate with that of a QWIP-FPA with the substrate. When the input IR beams illuminated only some pixels in the QWIP-FPA, the output signal from the FPA without GaAs substrate was about ten times higher than that from the FPA with a substrate. The output signals were almost same for both QWIP-FPAs where all pixels were uniformly illuminated by the IR inputs. These result indicate that the reflected beams from the GaAs back surface illuminate another pixels far away from the pixels which input beams strike first, when the FPA has a sufficiently thick GaAs substrate. We confirmed that the pseudo-random grating coupler for QWIP enhanced IR absorption by confining multi internal total reflections in one pixel, when thick substrates were removed.