Search

The transition-metal chromium-doped zinc oxide, ZnCrO, was successfully grown on an a -sapphire (110) substrate by radio frequency-plasma-assisted molecular beam epitaxy. The growth and optical property of ZnCrO were investigated. The growth mode changed from a 2-dimensional one to a 3-dimensional one with Cr doping. The ZnCrO films showed a textured morphology and the surface roughness of each was clearly reduced by increasing the amount of Cr doping. The X-ray diffraction peak intensity from ZnO (0002) became smaller as the Cr cell temperature was increased. A photoluminescence spectrum of ZnCrO was dominated by an excitonic peak around 3.36 eV. The transmission spectra showed that light wavelength shorter than 3.26 eV was removed. This was caused by Cr doping, and its losses caused by Cr doping. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In-doped Zn0.9Mg0.1O (0 ∼ 5 mol%) films on glass substrate were successfully grown by a spray pyrolysis method at 500 °C. The c-axis orientation became weak with increasing indium concentration. This indicated that In-doping caused the degree of crystallinity of the Zn0.9Mg0.1O films to decrease. Indium atoms could be acted as a donor type impurity because electrical conduction types in the undoped and In-doped films indicated all n-types and In-doping caused the resistivity to decrease and carrier concentration to increase. Furthermore, In-doping caused the number of nonradiative recombination centers to increase because photoluminescence intensity decreased with increasing indium concentration. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Undoped Zn1−X Mg X O poly crystalline films were successfully grown by a spray pyrolysis method at 500 °C. The samples indicated high quality because (0002) orientation was strongly observed in the X-ray diffraction (XRD) pattern below Mg content X = 0.3. A lattice constant of c axis decreased linearly with increasing Mg content, indicating that the lattice constant of c axis followed Vegard’s law. It was deduced that Mg atoms could be successfully substituted in Zn site from lattice constants and optical bandgap.

Transmission X-ray topography of the Ga-doped ZnTe crystals grown by the double crucible liquid encapsulated pulling (DCP) method is presented. The results show that dislocation density increases with growth direction from top to bottom. Photoluminescence (PL) intensity of the (A0, X) emission clearly decreases with the growth direction from top to bottom. Therefore, it is concluded that the PL intensity of the (A0, X) peak decreases with increasing dislocation density.

Undoped ZnO poly crystalline films were successfully grown by a spray pyrolysis method at 300–500∘C. The samples grown at 500∘C indicated high quality because the (0002) orientation was strongly observed in the X-ray diffraction (XRD) spectrum. Surface roughness reduced with increasing substrate temperature and indium concentration. Indium doping caused the resistivity to decrease and the carrier concentration to increase. Electrical conduction types in the undoped and In-doped ZnO films indicated all n-types. From these results, this indicated that indium atoms could act as a donor type impurity.

Cluster ion beam using ammonia as nitrogen source gas is applied to molecular beam epitaxial growth of GaN and AlGaN on sapphire(0001) as well as to nitridation of supphire substrate. The nitridation was succesful even at low as 600 °C by irradiating clusters with the peak size of ∼1000 [molecules/cluster] with the ion current density of 1 μA/cm2 at the acceleration energy of Ek=4–7 kV. Growth rate for GaN of ∼0.35 μm/h was obtaiend at 625–850 °C. Full width at half maximum of XRD rocking curve of GaN decreased with increasing the growth temperature and is ∼300 s at 925 °C. Results demonstrate that the use of cluster ion beam has great advantage of nitride thin film synthesis. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The sharp photoluminescence (PL) and optical-reflection spectra in the bandedge region of the high-quality nondoped ZnTe substrate (100) were observed at 4.2 K. Free exciton, associated with lower and upper polaritons (EXL and EXU) at 2.382 eV and 2.381 eV, respectively, were clearly observed. This meant that this substrate was high quality. The intensity of a bound exciton peak (2.375 eV), which is caused by a Zn vacancy, of a neutral acceptor decreased with an increase of the Zn vapor pressures.

We have found with deep level transient spectroscopy that chemical etching induced two isolated point defects, E1(0.26–0.31) and E2(0.20), in the near-surface region of n-type ZnSe single crystals. The etching effect on the traps in Br-doped ZnSe bulk crystals was studied by using 5% Br-methanol solution, and then the E1 trap density increased with the chemical etching time, and reached to maximum value of 5.3 × 1013 cm–3. The E1 trap was obtained in the samples etched in some kinds of acid solutions, Br-methanol solution, HCl, and HBr, but did hardly in NaOH alkali etchant. These results suggest that the E1 trap is induced during the chemical etching and diffuse into the interior of the crystals. From above etching effects, we propose a tentative identification that E1 and E2 traps arise from two kinds of hydrogen related defects.

Temperature dependence of the photoluminescence (PL) spectra in the high quality ZnTe substrate (111) was clearly observed. Three distinct kinds of peaks at 4.2 K were present at 2.362, 2.329 and 2.304 eV, which could be assigned to be the radiative recombination of the exciton bound to a neutral acceptor (I1), free to a neutral acceptor (FA) and with longitudinal optical (LO) phonon replicas, respectively. The bandgap energy (Eg) was estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy (12.9 meV). Therefore, the activation energy of the phosphor impurity was estimated to be 65 meV by subtracting the Eg to the peak energy of the FA emission band. Furthermore, it was clear that the PL emission was observed at 2.26 eV up to room temperature. This value was good agreement with the Eg at room temperature.

CuInS2 semiconductor crystals were successfully grown by a hot-press (HP) method at 400–700 °C for 1 h at growth pressures from 10 to 100 MPa. The samples were 20 mm in diameter. The samples grown at 700 °C were found to be of chalcopyrite structure, nearly stoichiometric and n-type by means of X-ray diffraction, electron probe microanalysis and thermoprobe analysis, respectively. A donor–acceptor pair emission band was observed in the photoluminescence spectra at 10 K, indicating that the samples had both donor and acceptor impurity types. It was also found that p-type CuInS2 crystals could be obtained by Sb doping. It was suggested that the Sb atoms in the S site might enhance p-type conductivity.

Room temperature photoluminescence (PL) and piezoelectric photothermal (PPT) spectroscopy were successfully performed on two different ZnS epitaxial layers grown by pyrolytic MOVPE on GaAs(100) by using diethyldisulphide (Et 2 S 2 ) as sulphur precursor at 390 °C and 435 °C. PL spectra allowed to identify several radiative recombination channels [namely, self-activated (SA) bands] associated to deep centres. Intense SA band occurred in PL and in PPT spectra recorded from ZnS grown at low temperature, indicating that RT radiative and non-radiative transitions both increases with decreasing the growth temperature below 400°C.

The photoluminescence (PL), optical reflectance, optical transmittance and piezoelectric photothermal spectra were successfully observed between liquid helium and room temperatures for high quality p-type P-doped ZnTe substrates. In the PL spectrum of the ZnTe substrate (1 × 10 18 cm -3 ), three distinct kinds of peaks which were due to the radiative recombination of the exciton bound to a neutral acceptor (I 1 ) and free to a neutral acceptor (FA) with longitudinal optical phonon replicas were present at liquid helium temperature. The bandgap energy was estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy. Furthermore, the activation energy of the phosphor acceptor impurity was estimated to be 65 meV. On the other hand, in the higher carrier concentration substrate (8 × 10 18 cm -3 ), the PL intensity was much smaller than in that of the lower carrier concentration substrate. This means that radiative recombination processes decrease with increasing carrier concentrations.

Self-assembled CdSe quantum dots have been fabricated by means of an alternate molecular beam supply. For the CdSe dot embedded in ZnSe and annealed at the growth temperature (250°C), the PL peak showed a blueshift with increase of the annealing period. This denotes that the size of embedded CdSe dots becomes small by the alloy formation with the surrounding ZnSe. The bare CdSe dots exposed to vacuum (without any molecular beam irradiation) evaporate, and the CdSe dots gradually changed to ZnCdSe dots. It was confirmed that the ZnCdSe dots were stable compared with pure CdSe dots.

CuInS 2 crystals were successfully grown by a hot-press method at 700°C for 1 h under high pressure (10–100 MPa). The size of each sample was 20 mm in diameter. All samples were shown to have chalcopyrite structures and were nearly stoichiometry and n-type by means of X-ray diffraction, electron probe microanalysis and thermoprobe analysis, respectively. However, the sample grown at 400°C also had a different phase (CuIn 11 S 17 ). The sample did not change phase with increasing temperature. CuInS 2 crystals could be obtained at 700°C. The samples had both donor- and acceptor-type impurities since donor–acceptor pair band emission was clearly observed in the photoluminescence spectra at 10 K.

The photoluminescence (PL), piezoelectric photothermal (PPT) and transmittance measurements were carried out at room temperature for ZnTe light emitting diode fabricated by the thermal diffusion of Al on p-type substrates. The same transmittance spectra were observed when the light was illuminated to both the Al-diffused side and the other sides without Al. No signals due to impurities were present in the transmittance spectra. However, a signal due to impurities was clearly observed at 2.15 eV in the PPT spectrum. This signal which had an activation energy of 110 meV was not seen in the PL spectrum. Therefore, it was deduced that the defect acted as a nonraditive recombination center.

Deep electron traps in Br-doped ZnSe crystals grown by the physical vapor transport method have been studied by using deep level transient spectroscopy. The Br-doped ZnSe crystal has an n-type conductivity and an electron concentration of 4.0 x 10 16 cm -3 in the as-grown state. Two isolated point defects are found with thermal activation energy of 0.20 and 0.31 eV. The Poole-Frenkel effect is not found in these traps. The density profile of the 0.31 eV trap as a function of the square of depth width better matched the Gaussian distribution function. This result suggests that the 0.31 eV trap is induced during the sample preparation, such as the chemical and/or the evaporation process.

The photothermal and photoluminescence spectroscopy for nondoped and chlorine (Cl)-doped ZnSe epitaxial layers were carried out at liquid nitrogen and room temperatures. We found two kinds of nonradiative carrier recombination centers which are Cl-related defects with activation energies of about 30 and 120 meV for the first time.

Photoluminescence (PL) and piezoelectric photoacoustic (PPA) spectra have been studied at low temperatures for nondoped and N-doped ZnSe epitaxial layers grown by molecular beam epitaxy. Deep emission bands observed in nondoped ZnSe are disappeared with nitrogen doping in the PL spectra. On the other hand, new nonradiative signals appear with nitrogen doping in the PPA spectra. This indicated that nonradiative carrier recombination centers are introduced by the nitrogen doping process in molecular beam epitaxy growth.

In situ RHEED observation of dot-assembling process during the MBE growth of CdSe on ZnSe(1 0 0) by alternate supply of Se and Cd beams was studied. Two-monolayer CdSe wetting layer is formed on Zn-terminated ZnSe(1 0 0). The third CdSe layer is formed once on the wetting layer, but this layer is unstable and immediately changes to CdSe dots and the CdSe dots develop by Ostwald ripening during Cd beam irradiation. Moreover, the CdSe dots sublimate while the uncapped CdSe dots are exposed to vacuum at the growth temperature. The dots development and sublimation were confirmed by the photoluminescence peak shift.

Piezoelectric photoacoustic (PPA) measurements of nondoped ZnSe epitaxial layers grown by molecular beam epitaxy (MBE) are carried out at room temperature. A distinct peak due to the bandgap of ZnSe and substrate GaAs is clearly observed. The deep level with activation energy of about 0.65 eV is not successfully resolved from the nonradiative electron transition point of view. The PPA signals of nondoped ZnSe/GaAs thin film grown by MBE are successfully observed for the first time.

Piezoelectric photoacoustic (PPA) measurements of nondoped ZnSe epitaxial layers grown by molecular beam epitaxy (MBE) are carried out at room and liquid nitrogen temperatures. Distinct peaks due to the band-gaps of ZnSe and the substrate GaAs are clearly observed. Five PPA peaks are observed in the nondoped ZnSe thin films with VI/II ratio of 12 at liquid nitrogen temperature for the first time. This indicates that five types of intrinsic defects exist in ZnSe epitaxial layers. The obtained energies of these defects are 1.20 ± 0.01, 0.97 ± 0.03, 0.65 ± 0.03 and 0.06 ± 0.01 eV, respectively.

Wurtzite—CdSe epitaxial layers on GaAs(1 1 1)A and B substrates were grown by molecular beam epitaxy using elemental Cd and Se beams. A distinct RHEED oscillation of a specular spot intensity was observed during ALE growth of CdSe on GaAs(1 1 1)B, but the oscillation on GaAs(1 1 1)A was poor. A GaAs(1 1 1)B substrate is superior to GaAs(1 1 1)A substrate for the layer-by-layer growth of wurtzite CdSe. The activation energy of Se desorption from the GaAs(1 1 1)B surface was determined as 1.2 ± 0.1 eV by analyzing the temporal specular beam intensity. The crystallographic and luminescence properties are also presented.

Li-doped ZnSe films have been grown on misoriented GaAs(0 0 1) substrates by molecular-beam epitaxy to investigate the effects of step density on the electrical and the optical properties. We found that the net acceptor concentration in Li-doped ZnSe has a strong relationship with the step density on the GaAs substrates and the misorientation direction. The net acceptor concentration on the off-substrate towards [1 1 0] is higher than that on the off-substrate towards [1 1 0]. The difference in Li concentration can be explained by the doping efficiency and the generation of donor-like centers.

Intensity of a photoluminescence line named I V observed at the near band-edge region for undoped ZnSe/GaAs is found to increase exponentially with the time duration of photoexcitation. The rise time is unexpectedly long, i.e. several hundreds of seconds under low excitation intensity and at low temperatures, and decreases with increasing excitation intensity or temperature. Experimental results lead us to suggest that the luminescence center responsible for the I V line is the Se vacancy which captured one or two electrons.

Sharp and semicircular patterns were observed in RHEED during the MBE growth of Li- or Na-acceptor doped ZnSe and ZnS on GaAs(001). The radius and the separation between the diffraction circles vary with the change of the azimuth of the incident electron beam. Calculated diffraction patterns assuming that Li or Na atoms are arrayed one-dimensionally along the [110] direction of the crystal axis are in good agreement with the experimental results. We conclude that Li or Na atoms are incorporated at the [110] terrace steps, which prevents the further growth from the step edge.

Li-doping of ZnS/GaAs layers by means of molecular beam epitaxial (MBE) growth using post-heated molecular beams is reported. Low-temperature photoluminescence measurements and secondary ion mass spectrometry have clarified that the Li atoms act as acceptors and donor Ga is incorporated from the substrate. It is also found that under higher doping levels the growth rate is considerably reduced. Together with the reflection high-energy electron diffraction (RHEED) results, this behavior is attributed to some interaction between Li and S atoms on the growing surface.

The donor species in Li acceptor-doped ZnSe epilayers grown on GaAs substrate are determined by means of the secondary ion mass spectroscopy (SIMS). The compositional depth profile by the SIMS points out that a large amount of Ga atoms are incorporated into the ZnSe epilayer regardless of doping. In doped epilayers, the Ga atoms are concentrated at the region where the Li atoms are doped. We conclude that Ga atoms originating from the droplets due to the thermal cleaning of the substrate are most likely the major donor species.

We report successful growth of ZnS on GaAs(001) using elemental sulfur and zinc sources. In order to use S, which has a very high vapor pressure for molecular beam epitaxy, a new type of effusion cell is developed. The cell is composed of the effuser and post-heating zones. Not only the control of vapor pressure, but also bakeout of the MBE chamber could be achieved. ZnS/GaAs could be grown at temperatures as low as 150°C using S beam post-heated at 210°C and conventional Zn beam. Low temperature photoluminescence shows that high quality ZnS films are grown.

Temperature dependence of the piezoelectric photoacoustic (PPA) and photoluminescence (PL) spectra are clearly observed between liquid helium and room temperatures for nondoped ZnSe bulk crystals and epitaxial layers. In both samples, the PPA signals are clearly observed at room temperature. However, any PL emissions are not observed at room temperature. Two kinds of recombination processes, i.e., both radiative and nonradiative processes are observed at 80 K in the bulk crystals.

Evidence is presented for strong correlation between new donors and rodlike defects generated at 650 °C in phosphorus‐doped, carbon‐lean Czochralski silicon preannealed at 450 °C. It is proposed that there are, in general, several types of new donors depending on experimental conditions, and one type of new donor, which is generated preferentially under the above special condition, arises from rodlike defects.

We have evaluated hydrogen and deuterium diffusivities in silicon below room temperature (220–270 K) by analyzing the kinetics of photoinduced dissociation of a chemical etching introduced hydrogen (deuterium)–carbon complex. Under sufficiently strong illumination, the annihilation rate of the complex was proportional to the phosphorus density, indicating that the rate‐determining step is the diffusion of hydrogen (deuterium) to phosphorus atoms. Applying the diffusion‐controlled reaction theory, we have evaluated the diffusion coefficients as 7×10−2exp(−0.54 eV/kT) cm2 s−1 for hydrogen and 5×10−3exp(−0.49 eV/kT) cm2 s−1 for deuterium, being in good agreement with the extrapolation of the high‐temperature diffusion data of A. Van Wieringen and N. Warmoltz [Physica 22, 849 (1956)].

We have found by resistivity measurements and deep‐level transient spectroscopy that four kinds of oxygen‐related shallow donors, OD1–OD4, successively appear around 500 °C in the time region 10–106 min and carbon greatly affects their generation behavior in Czochralski silicon. The formation of OD2 and OD3 donors was enhanced by the presence of carbon, while that of OD1 and OD4 donors was retarded. The OD1 donor is identified as the so‐called thermal donor that consists of several kinds of double donors observed so far by infrared studies. The OD2 donor is a new kind of donor discovered recently by us. OD3 and OD4 donors, which were formed by very prolonged annealing around 500 °C, are suggested to have correlations with so‐called new donors normally generated above 600 °C. The origins of these oxygen‐related donors are discussed in relation to oxygen precipitation.

This paper demonstrates a unique action of hydrogen on defects and impurities in semiconductors. Hydrogen injected into n-type Si by chemical etching or hydrogen plasma not only pas-sivatcs phosphorus but also electrically activates carbon by forming a H-C complex acting as an electron trap E3 (0.15). A model of the structure and electronic state of the H-C complex is proposed on the basis of available experimental data on the properties of the complex. The diffusion coefficient of isolated hydrogen below 300K is evaluated from its diffusion process to phosphorus after the photoinduccd dissociation of the H-C complex. Some differences in hydrogen diffusion between chemically etched and plasma hydrogenated crystals arc discussed.

We have found with deep level transient spectroscopy (DLTS) measurement two hole traps with a thermal activation energy of 0.20eV and 0.48eV in Se-annealed CuInSe2 grown by a travel heater method (THM). The applications of a reverse and a forward bias under cooling the sample for DLTS cause an increase and a decrease of the trap density of 0.20eV hole trap. It suggests the 0.20eV trap is a metastable defect, the charge state of the trap changes by applying an electric field at low temperature region(

We report the details of a post-heating cell developed to obtain high-velocity molecular beams. Zn and Se molecular beams generated from this cell are evaluated by measuring deposition rate by means of a quartz thickness monitor. The observed deposition rates are analyzed by means of a simple model for the operation of an ionization gauge. We found that Zn atoms have a thermal velocity determined by the temperature at the post-heating zone, and that Se molecules are thermally cracked.

Epitaxial layers of ZnSe and ZnS were grown by molecular‐beam epitaxy at temperature as low as 150 °C by using postheated molecular beams generated from elemental materials. To obtain postheated molecular beams as well as to control flux intensity for materials with very high vapor pressure, new types of effusion cells are developed. Experiments under the different temperature and molecular‐beam conditions clarified that the enhancement of the surface migration of Zn atoms is essential to the lowering of the growth temperature.

We have discovered a new family of oxygen‐related double donors [new thermal donors (NTD’s)] generated around 450 °C in phosphorus‐doped Czochralski silicon by combining deep‐level transient spectroscopy with Hall measurements. This new family was well distinguished from the normal family of thermal donors (TD’s) currently studied so far. Our results have shown that both families of thermal donors exhibit qualitatively the same kinetic behavior. Namely, as the annealing time increases, their ionization energy of levels continuously decrease with their densities increasing until the maxima and then become constant with their densities decreasing. However, there are significantly quantitative differences between the both families; NTD’s have shallower levels, considerably smaller generation rates, and higher thermal stability than TD’s. Sufficiently prolonged annealing for more than 105 min around 450 °C or short donor‐killing annealing for 20 min at 650 °C completely annihilates TD’s, leaving only NTD’s, o...

We have studied the effects of low-temperature preannealing and carbon on new donor formation at 650°C in phosphorus-doped Czochralski (CZ) silicon by deep-level transient spectroscopy (DLTS). In not preannealed carbon-lean samples, only a weak continuous DLTS spectrum often reported so far was observed. The intensity of this broad feature became significantly stronger in not preannealed carbon-rich samples. On the contrary, preannealed samples showed no such continuous spectra but two new DLTS peaks arising from shallow donor levels. Carbon enhanced the low-temperature peak, but retarded the high-temperature one. The latter peak is in strong correlation with the rodlike defect.