Toru Aoki

ZnO diode fabricated by excimer-laser doping

A ZnO diode was fabricated by using a laser-doping technique to form a p-type ZnO layer on an n-type ZnO substrate. A zinc-phosphide compound, used as a phosphorous source, was deposited on the ZnO wafer and subjected to excimer-laser pulses. The current–voltage characteristics showed a diode characteristic between the phosphorous-doped p-layer and the n-type substrate. Moreover, light emission, with a band-edge component, was observed by forward current injection at 110 K.

Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation

The dependence of the structural, photoluminescent and cathodoluminescent properties of ZnO thin films deposited by electron beam evaporation on the preparation conditions has been investigated. Both as-deposited and annealed thin films deposited at substrate temperatures higher than 200°C showed c-axis orientation, and their crystallinity was improved with increasing annealing temperature. The films showed the emission with a peak at around 510 nm in photoluminescence (PL) and cathodoluminescence (CL) except for the film annealed at 800°C in air. The emission seems to be well-known blue-green emission due to ZnO:Zn phosphor. The strong green emission with a peak at around 540 nm was obtained from the film annealed at 800°C in air. The origin of the emission is not understood. The film showed CL luminance of about 60 cd/m2 under excitation of 2 kV, 400 μA/cm2. Moreover, it showed CL under excitation even at 250 V without charging-up.

Excimer laser doping techniques for II–VI semiconductors

Abstract Excimer laser doping technique has been utilized to obtain a heavy impurity doping in II–IV semiconductors which are considerably difficult materials to achieve bipolar conductivity and to obtain good ohmic contact on them during device fabrication. p-type heavy dopings were examined on CdTe, ZnSe and ZnO using dopant atoms such as Na, K, Sb, or P and diffusing them inside the crystals by irradiating with an excimer laser. Heavily doped layers with the resistivity in the range from 10 −2 –10 −3 xa0Ωxa0cm and the hole concentration of 10 17 –10 18 xa0cm −3 could be obtained. The carrier mobility in those doped p-type layers was in the range from 10–100xa0cm 2 /V-s. n-Type doping of CdTe was also investigated using In as a dopant. A highly conductive n-type layer with resistivity, electron concentration and mobility of 5×10 −3 xa0Ωxa0cm, 8.9×10 18 xa0cm −3 and 140xa0cm 2 /V-s, respectively, was successfully obtained. Finally, a CdTe p–i–n diode was fabricated which showed a very good potentiality to be used as a nuclear radiation detector.

Surface processing of CdTe compound semiconductor by excimer laser doping

Abstract Laser processing technique has been developed to achieve heavy impurity doping in II–VI compound semiconductor such as CdTe which is excellent material for application in high energy flux detector because of its high absorption coefficient and energy resolution due to large atomic number and high carrier mobility. At first, transient temperature increment in CdTe due to excimer laser radiation will be discussed using computer simulation data. Using KrF excimer laser with a pulse width of 20 ns and a wavelength of 248 nm, one shot of laser of energy 80 mJ/cm 2 increases the surface temperature of CdTe more than 1000°C, the melting point of CdTe. As the laser pulse is very short and the penetration depth of UV laser light is very short, the depth of processed and modified layer is limited to few hundred angstroms. Secondly, we have experimentally studied the laser doping process for the application to detectors of high energy flux. Resistivities of CdTe surface processed by laser doping drastically decreased from 10 5 to 10 −1 Ω cm. p–i–n diode was thus fabricated on a high resistivity single crystal CdTe using excimer laser doping on one side for p-layer and n-type CdTe grown epitaxially on the other side. The p–i–n diodes resulted showed promising characteristics and high energy sensitivity. This technique is also promising to form ohmic contacts in other II–VI semiconductor devices. Because we can adapt this process at the final stages during device fabrication and also the laser effect is limited to a very thin layer, hence there will be no influence in the bulk characteristics of the device.

Improved spectrometric performance of CdTe radiation detectors in a p-i-n design

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150u200a°C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25u200a°C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.

FABRICATION AND PERFORMANCE OF P-I-N CDTE RADIATION DETECTORS

Abstract We report on the fabrication and performance of CdTe radiation detectors in a new p–i–n structure which helps to reduce the leakage current to a minimum level. Chlorine-doped single-crystal CdTe substrates having resistivity in the order of 10 9 xa0Ωxa0cm were used in this study. Iodine-doped n-type CdTe layers were grown homoepitaxially on one face of each crystals using the hydrogen plasma-radical-assisted metalorganic chemical vapor deposition technique at low substrate temperature of 150°C. Indium electrode was evaporated on the n-CdTe side while a gold electrode on the opposite side acted as a p-type contact. Detectors thus fabricated exhibited low leakage current (below 0.4xa0nA/mm 2 at 250xa0V applied reverse bias for the best one) and good performance at room temperature. Spectral response of the detectors showed improved energy resolution for Am-241, Co-57, and Cs-137 radioisotopes. Detectors were further tested with X-ray photons of different intensities for their potential application in imaging systems and promising responses were obtained.

Epitaxial growth of CdSeTe films by remote plasma enhanced metal organic chemical vapor deposition

Abstract The epitaxial growth of cadmium selenium telluride (CdSeTe) films has been studied using a remote plasma-enhanced metal organic chemical vapor deposition (RPE–MOCVD) method. Hydrogen radicals generated by inductively coupled rf remote plasma were introduced into the reaction region during the deposition. Therefore, these film growth procedures could be carried out at the low substrate temperatures below 200°C. In the CdSeyTe1−y deposition, the composition ratio y has been obtained in the range 0 to 1 when the ratio of the Group VI source monomers was varied. The role of hydrogen radicals is considered as to decompose the metal organic source materials and to help the surface reaction for the film formation. The n-type CdSeTe layers were also obtained using n-butyliodide as a dopant source. The resistivity and carrier concentration in the CdSeTe epitaxial layer were about 3×10−2xa0Ωxa0cm and 2×1018xa0cm−3, respectively. It is concluded that the CdSeTe epitaxial growth has a high potential as n-type material of the CdZnTe diode to fabricate the high-energy radiation detector.

Growth and doping studies of CdTe epilayers on GaAs substrates by low-pressure plasma-radical-assisted metalorganic chemical vapor deposition

Abstract Heteroepitaxial layers of CdTe were grown on GaAs substrates by the hydrogen-radical-assisted metalorganic chemical vapor deposition (MOCVD) technique at a low pressure and temperature. Dimethylcadmium and diethyltelluride were used as the source materials and hydrogen radicals produced in a remote plasma source by applying inductively coupled RF power were introduced into the reaction chamber. The growth was carried out in the substrate temperature range of 150–300°C, at a pressure of 0.2xa0Torr. The grown films, undoped, have high resistivities in the order of 10 5 xa0Ωxa0cm for the entire growth range. Gas-phase iodine doping to obtain n-type conductivity was carried by utilizing n-butyliodine as a dopant precursor, whereas p-type doping was achieved by employing nitrogen plasma radicals produced by nitrogen or ammonia gas in addition to hydrogen during the film growth. Highly conductive n- and p-type CdTe films with carrier concentration in the order of 10 18 xa0cm −3 and mirror-like surface were obtained.

A New Fabrication Technique of CdTe Strip Detectors for Gamma-Ray Imaging and Spectroscopy

Diode type CdTe strip detectors were fabricated in a new and simple technique for imaging and spectroscopy applications. On a high resistivity p-like single crystal CdTe wafer, n-type strips were formed by evaporating a thin indium layer on the crystal through a metallic shadow mask and then diffusing it inside the crystal by irradiating with an excimer laser. On the opposite side of the crystal a gold electrode was evaporated on the whole surface. The current-voltage characteristics of the detectors exhibited diode-like property with a low value of reverse bias leakage current from the individual strips. The spectral response from all strips was very good and uniform with typical energy resolutions of 2.5 keV and 3.6 keV FWHM at 59.5 keV peak of Am-241 and 122 keV peak of Co-57 radioisotopes, respectively for radiation detection tests performed at room temperature. Details about the detector fabrication and the spectral results that demonstrate the imaging and spectroscopy capabilities of these detectors are presented.

Fabrication of CdTe strip detectors for imaging applications

Abstract p–i–n CdTe strip detectors were fabricated in a new and simple technique for their probable applications in astrophysics and medical imaging. The detector has an iodine-doped n-CdTe epitaxial layer followed by aluminum metallization on the Te-face of (1xa01xa01) oriented CdTe crystal, as an anode. The cathode consists of the p-type segmented regions formed on the crystal by the diffusion of dopant atoms from an alkaline metal compound through a patterned shadow mask using the excimer laser radiation. Afterward, the gold electrode is evaporated through the same shadow mask to make the strip cathodes. In a different way, we also studied the laser ablation process for the selective area etching by irradiating the laser through a mask. Details of these fabrication processes for making strip detectors and results on laser-assisted etching process will be presented.