bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of bound exciton complexes in ZnO
Authors: Strassburg M. , Rodina A. , Dworzak M. , Haboeck U. , Krestnikov I.L. , Hoffmann A. , Gelhausen O. , Phillips M.R. , Alves H.R. , Zeuner A. , Hofmann D.M. , Meyer B.K.
Published: February 2004

An identification of shallow bound exciton centers in ZnO is presented based on magneto‐optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I4, I6, I8 and I9 exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the Γ7 symmetry of the upper valence band. The presence of two‐electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I4, I6, I8 and I9 lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High‐quality ZnO epilayers grown on Zn‐polar ZnO substrates by plasma‐assisted molecular beam epitaxy
Authors: Kato Hiroyuki , Sano Michihiro , Miyamoto Kazuhiro , Yao Takafumi
Published: February 2004

High‐quality ZnO epilayers have been grown on Zn‐polar ZnO substrates by plasma‐assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O‐rich, the growth mode changed from three‐ to two‐dimensional growth and the line widths of (0002) and (10–10) ω‐rocking curves showed dramatic narrowing from 471 to 42 arsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device‐grade MOCVD‐grown GaN. Moreover, A‐, B‐excitons (FEA, FEB), and the n = 2 state of FEA at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low‐temperature (4.2 K) photoluminescence spectrum of ZnO grown under O‐rich flux conditions. Our results show that growth under high O‐rich flux conditions is required to produce high‐quality Zn‐polar ZnO films. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Extrinsic doping effect in the fabrication of CIGS and CIGSS thin film solar cells
Authors: Ramanathan K. , Pankow J. , Asher S.
Published: February 2004

This paper describes the effect of Cd and Zn impurities in the formation of photovoltaic junctions using CuInSe2‐based alloy thin‐film absorbers. We compare the properties of solar cells fabricated by using CdS window layers with those obtained by Cd or Zn treatment from the liquid or vapor‐phase environments. Efficient solar cells are obtained for the latter two cases, and their properties are comparable to those with CdS window layers. The results are interpreted in terms of the ability of Cd and Zn to produce n‐type doping in the surface region of the absorbers. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Development of CdxZn1−xTe alloy thin films for tandem solar cell applications
Authors: Dhere Ramesh , Gessert Tim , Zhou Jie , Pankow Joel , Asher Sally , Moutinho Helio
Published: February 2004

We report our development efforts for CdxZn1−xTe alloy thin films for tandem solar cells. We used three aspects of CdTe solar cell fabrication considered key to high performance: high‐temperature deposition, post‐deposition CdCl2 heat treatment, and use of O2 ambient during device processing. We used bilayers of CdTe and ZnTe as the basis for the formation of CdxZn1−xTe alloy. Use of oxygen at any stage of the processing proved to be problematic due to the formation of ZnO, which resulted in two‐phase mixture of CdTe and ZnO. These results can explain the poor performance of CdxZn1−xTe alloy thin‐film solar cells reported by several groups. We carried out oxygen free processing of the bilayer stacks and post‐deposition heat treatments. Heat treatment in the presence of CdCl2 vapor resulted in the interaction between ZnTe and CdCl2 and the loss of Zn in the form of ZnCl2. Heat treatments of the stack in He ambient resulted in the interdiffusion of the two layers. The composition profiles of the intermixed alloy region were influenced mainly by temperature and the time of the treatment. It is possible to obtain either uniform or graded alloy profiles by adjusting the two parameters. It is possible to adjust the composition of the resuting alloy by adjusting the thickness ratio of CdTe and ZnTe layers. We discuss suitability of the alloy formation using the CdTe/ZnTe bilayers for the top cell fabrication of two‐junction tandem solar cells. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of undoped ZnO layers grown by molecular beam epitaxy towards biosensing devices
Authors: Ogata K. , Komuro T. , Hama K. , Koike K. , Sasa S. , Inoue M. , Yano M.
Published: February 2004

Undoped ZnO layers grown by molecular beam epitaxy (MBE) were investigated aiming at biosensing devices based on organic/inorganic hybrid structures. Clear observation of free exciton emission in low temperature photoluminescence (PL) spectra and electron mobility as high as 114 cm2/Vs at room temperature indicate that the quality of the ZnO layers is applicable for biosensors. Surface bonding of the ZnO the layers, which is one of the key issues for hybridization, was examined by means of xray photoelectron spectroscopy (XPS). The XPS spectra of O 1s revealed that formation and desorption of hydroxyl (OH) bonds can be controlled by air exposure and thermal treatments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Dephasing of interacting σ+ and σ− excitons in a ZnSe single quantum well
Authors: Wagner H. P. , Tripathy S.
Published: February 2004

We investigate coherences between σ+ and σ− exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three‐pulse degenerate four‐wave mixing using circularly polarized 100 fs pulses. Intensity dependent measurements are performed to study the influence of exciton–exciton scattering on their dephasing rates. The experiments reveal nearly equal zero‐density dephasing rates for coherences between σ+ and σ− heavy‐ and/or light‐hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton‐exciton scattering destroys the coherence between heavy‐ and light‐hole excitons about three times more effectively than the coherence between σ+ and σ− heavy‐hole excitons. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Magnetotransport and magnetic properties of p‐Zn1− xMnxTe:N – Carrier‐induced ferromagnetism
Authors: Nam K.‐T. , Kuroda S. , Kumekawa T. , Ozaki N. , Takita K.
Published: February 2004

The magnetic properties of nitrogen‐doped p‐Zn1− xMnxTe grown by MBE were studied through the magnetotransport measurements. In a sample of p‐Zn1− xMnxTe with x = 0.034 and p = 4.7 × 1019 cm−3, a hysteresis loop was observed below 2.5 K in the Hall resistance and the magnetoresistivity, which indicates the ferromagnetic transition. From the plot of (RHall/Rsheet)2 against B/(RHall/Rsheet) at several temperatures, which corresponds to the so‐called Arrott plot, we can estimate the Curie temperature (Tc) of the sample as about 3 K, suggesting that this sample has the higher Tc than ever reported. We also demonstrated, by means of light irradiation, that the strength of ferromagnetic interactions can be controlled by changing the hole concentration. An increase of the ferromagnetic behaviours such as a small increase of the coercive field was observed under light irradiation. This result strongly supports that the ferromagnetic transition in Zn1− xMnxTe is the carrier‐induced one. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Confinement‐induced enhancement of spin interactions in Mn‐doped II–VI semiconductor nanocrystals
Authors: Bhattacharjee A. K. , Pérez‐Conde J.
Published: February 2004

The effects of sp–d exchange interactions on the optical properties of quantum dots containing a single Mn ion are investigated within the effective mass formalism. The quantum‐mechanical eigenvalue problem of an electron‐hole pair in interaction with the Mn spin is solved analytically. In zinc‐blende semiconductors such as CdTe the fundamental absorption line splits into six components, whose relative intensities strongly depend on the Mn spin orientation with respect to the polarization of light, suggesting the possibility of optical detection of spin. In a magnetic field the splitting between the strong components of the σ− and σ+ spectra rapidly attains a saturation value close to the overall zero‐field splitting, which is inversely proportional to the nanocrystal volume. Typically, it is an order of magnitude larger than the saturation Zeeman splitting in the corresponding bulk diluted magnetic semiconductor. The numerical results for ZnSe:Mn nanocrystals show a good agreement with the published experimental data. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Zinc oxide thin films synthesized by metal organic chemical reactions
Authors: Yasuda Takashi , Segawa Yusaburo
Published: February 2004

High quality ZnO films on sapphire substrates were synthesized using metal organic chemical reactions, such as metal organic chemical vapour deposition (MOCVD) and metal organic decomposition (MOD) method. Stable solid source of Zn‐acetylacetonate was used for MOCVD, and the sol‐gel method from Zn‐acetate, 2‐methoxyethanol and 2‐aminoethanol, and a commercial MOD source from Symetrix Co. were used for MOD. Dominant excitonic emission was obtained from these layers synthesized by these simple and economical methods, which are expected to develop powerful techniques for functional oxide fabrications. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Slow mode degradation mechanism of ZnSe based white LEDs
Authors: Adachi Masahiro , Ando Koshi , Abe Tomoki , Tsutsumi Sueyuki , Inoue Noboru , Kasada Hirofumi , Katayama Koji , Nakamura Takao
Published: February 2004

This paper presents slow mode degradation mechanism of ZnSe‐based white LEDs, which is induced by point defect. The white LEDs are very high quality devices, of which the optical power at a forward current of 20 mA is 8.3mWand the luminous efficiency is 28 lm/W, which is comparable to GaN‐based white LEDs in the market. It is proven that microscopic point defects in the devices, detected as deep hole traps of the H1 center (Ev + 0.6 eV) and H0 center (Ev + 0.8 eV) in p‐type ZnMgSSe cladding layer, are responsible for the slow‐mode degradation. These point defects, related to nitrogen complex centers, are very electrically active under device operation. An important insight of the H0 center are found that the mobility of the defect is extremely high, and then it induces marked interaction with macroscopic defects. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Time‐ and bias‐dependent effects in ZnCd(Mg)Se blue and red quantum well light emitting diodes studied by cathodoluminescence
Authors: Nikiforov A. Yu. , Cargill III G. S. , Guo S. P. , Tamargo M. C.
Published: February 2004

Luminescence variations in MBE‐grown blue and red ZnCd(Mg)Se/InP QW LEDs have been studied by time‐resolved and bias‐dependent cathodoluminescence (CL) spectroscopy. CL from the red LED reversibly increased under forward bias and decreased under negative bias during several complete biasing cycles. CL from blue LEDs was bias dependent only during the first few cycles. The irreversible effects were QW CL intensity decreases after electron bombardment for the red LED and QW CL intensity enhancements seen as bright regions in CL images and dark regions in SE micrographs for blue LEDs. For the red LED, the bombarded regions were dark in CL images and had a dark dot and halo contrast in SE micrographs. The enhancements of CL intensity in the blue LEDs may result from buildup of internal fields due to persistent charge trapping in preexisting or electron bombardment created defects. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Exciton localization in MgxZnyCd1−x−ySe alloy
Authors: Maksimov O. , Wang W. H. , Samarth N. , Muñoz M. , Tamargo M. C.
Published: February 2004

We report photoluminescence and reflectivity measurements of MgxZnyCd1−x−ySe epitaxial layers (0 < x < 0.53) grown by molecular beam epitaxy on InP (100) substrates. Significant emission line broadening, increase in activation energy and Stokes shift are monitored with increasing Mg content. For MgxCdyZn1−x−ySe samples with large Mg content (x > 0.3), we observe an anomalous temperature dependence of both the emission energy and line broadening. This behavior is assigned to the emission from localized states. Different mechanisms of carrier localization are discussed and exciton localization on statistical CdSe clusters is proposed to be the most likely one. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
CBD‐Cd1−xZnxS thin films and their application in CdTe solar cells
Authors: Zhou J. , Wu X. , Teeter G. , To B. , Yan Y. , Dhere R. G. , Gessert T. A.
Published: February 2004

Composition, optical properties, structure properties, and surface morphology of thin films of Cd1−xZnxS (x ≤ 10%) prepared by chemical bath deposition (CBD) are reported. The best cell efficiency was 15.7%. It used a Cd1−xZnxS window layer, was confirmed by NREL (Voc = 840.1 mV, Jsc = 24.81 mA/cm2, and FF = 75.55%), and had a CTO/ZTO/Cd0.92Zn0.08S/CdTe cell structure. The use of low zinc concentration in the Cd1−xZnxS layer and interdiffusion between Cd1−xZnxS and CdTe layers were assumed to be the reasons for maintaining high Voc and FF. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Magneto‐optical spectroscopy of gated p‐doped CdMnTe quantum wells
Authors: Boukari H. , Bertolini M. , Ferrand D. , Cibert J. , Tatarenko S. , Kossacki P. , Gaj J. A.
Published: February 2004

We describe the magneto‐photoluminescence of (Cd,Mn)Te/(Cd,Zn,Mg)Te quantum wells containing a 2D hole gas . The hole population can be tuned either optically in p‐i‐p structures, by above barrier illumination, or electrically by applying a bias voltage of a few volts on a p‐i‐n structure. We identify the photoluminescence at low field as due to the positively charged exciton X+, which is destabilized at a given value of the Zeeman splitting, due to its crossing with a state involving a triplet configuration of the trion hole pair. The X+ looses its intensity in favor of a double line which exhibits features associated to band‐to‐band transitions at k = 0 and k = kF (wave vector at Fermi energy). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Time resolved photoluminescence studies of Zn–Se–Te nanostructures with sub‐monolayer quantities of Te grown by molecular beam epitaxy
Authors: Gu Y. , Kuskovsky Igor L. , van der Voort M. , Neumark G. F. , Zhou X. , Muñoz M. , Tamargo M. C.
Published: February 2004

We investigate triple‐delta‐doped ZnSe:Te samples grown with different Te/Zn flux ratios using time‐resolved photoluminescence (TRPL). We show that the properties of the TRPL of both samples are consistent with the presence of quantum islands with a type‐II band alignmrnt. Moreover, from the comparison of the PL, we show that higher Te/Zn flux ratio during the growth leads to the formation of larger quantum islands. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
ZnTe single crystal growth by the liquid encapsulated pulling method
Authors: Asahi T. , Yabe T. , Sato K.
Published: February 2004

Large diameter ZnTe single crystals were grown by the liquid‐encapsulated pulling method using a double crucible. Crystals of around 80 mm in diameter and 40 mm in length were reproducibly obtained by this method. Both 〈100〉 and 〈110〉 crystals were successfully grown. The strong facet growth was observed when the shoulder part of the bulk was formed. Rectangular facet growth was observed in 〈100〉 growth and hexagonal facet growth was observed in 〈110〉 growth. The etch pit density of the grown crystal was lower than 10000 cm−2 and the strain in the crystal was lower than that of modified liquid encapsulated Kyropoulos method. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
ZnCdSe quantum structures by (110)‐cleaved‐edge overgrowth: MBE growth and μ‐PL characterization
Authors: Titova L. V. , Cywiński G. , Kutrowski M. , Wojtowicz T. , Liu X. , Furdyna J. K. , Dobrowolska M.
Published: February 2004

Cleaved edge overgrowth has been used to fabricate zero‐ and one‐dimensional ZnCdSe nanostructures in a ZnSe matrix. Polarization‐dependent micro‐photoluminescence results indicate that 0‐D structures (quantum dots) are present in the entire overgrowth region due to (110) interface roughness, while 1‐D structures (quantum wires) appear only in the region where quantum wells grown on (001) and (110) surfaces intersect. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical spin injection and tunneling in asymmetric coupled II–VI quantum wells
Authors: Nawrocki M. , Kłopotowski Ł. , Suffczyński J.
Published: February 2004

We give an overview of recent results concerning exciton and carrier tunneling and relaxation in II–VI asymmetric double quantum wells structures with semimagnetic (diluted magnetic) wells or barriers. We undertake the issue of the spin relaxation and injection in the tunneling process and we discuss the dependence of the tunneling efficiency and spin injection on the energy detuning between the initial and final state of the process. The fast tunneling of excitons as whole entities was observed with efficiency strongly dependent on the barrier width, exciton detuning and the relation between detuning and optical phonon energy. In the energy transfer processes an important role of the charged excitons formation is observed. As a result of high sensitivity of energy levels in semimagnetic semiconductor to magnetic field, the thermodynamical redistribution of neutral and charged excitons dependence in magnetic field is observed and must be taken into account in the analysis. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rotation‐domains suppression and polarity control of ZnO epilayers grown on skillfully treated c‐Al2O3 surfaces
Authors: Yoshikawa Akihiko , Wang Xinqiang , Tomita Yosuke , Roh Ok‐Hwan , Iwaki Hiroyuki , Ishitani Yoshihiro
Published: February 2004

Rotation domains are easily and often introduced into ZnO epilayers grown on c‐Al2O3 substrate, resulting in degraded epilayer quality. In this paper, effects of the sapphire surface properties on the epitaxy of ZnO were studied; the surface was carefully treated by high temperature thermal cleaning (TC), atomic hydrogen (H*), oxygen radical (O*) treatments, decoration by a few monolayers gallium and nitridation, and then the ZnO epilayers grown on these surfaces were characterized. Three kinds of rotation domains were observed in total and the sub‐domains were completely suppressed by Ga decoration and sapphire nitridation. The effect of the above treatments on polarities of ZnO epilayer was also studied. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of CdSe/ZnSSe quantum dot ordering by grazing incidence small angle X‐ray scattering
Authors: Schmidt Th. , Clausen T. , Falta J. , Bernstorff S. , Alexe G. , Passow T. , Hommel D.
Published: February 2004

The spatial correlation of CdSe quantum dots in stacked CdSe/ZnSSe quantum dot layers has been investigatedusinggrazing incidence small angle X‐ray scattering. Lateral and vertical ordering has been found by the presence of satellite spots, and its dependence on the number of quantum dot layers and on the spacer layer thickness has been studied. Significant ordering occurs, if the number of quantum dot layers is not too small, and if the ZnSSe spaces thickness is not too large. These findings are interpreted in terms of a strain‐driven, self‐organized ordering process. In addition, an anisotropy of two‐fold symmetry has been observed, with the weakest correlation signal along 〈110〉. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Magnetic field switching of spin injection and spin coherence in magnetic quantum structures
Authors: Ghali M. , Kossut J. , Janik E. , Teppe F. , Scalbert D.
Published: February 2004

We report on spin injection and spin dynamics of free carriers in II–VI hybrid magnetic quantum structures. As a model structure for spin injection, a MBE‐grown Cd0.96Mn0.04Te/Cd0.98Mg0.02Te/CdTe QW was studied. The Cd0.96Mn0.04Te acts as a spin aligner while a thick intermediate spacer (1000 Å) of Cd0.98Mg0.02Te can be also used to detect the carrier spins just right after their crossing the interface (in contrast to usual detection done at the QW optical transition wavelength). All‐optical CW and time‐resolved techniques were used to monitor the spin transport and spin relaxation in our structure. By tuning the spin aligner‐spacer layer band offset with an external magnetic field, we demonstrate that spin injection into the non‐magnetic CdMgTe region may be switched on and off by the field. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Quantum structures in Zn–Se–Te system containing submonolayer quantities of Te
Authors: Kuskovsky Igor L. , Gu Y. , van der Voort M. , Neumark G. F. , Zhou X. , Muñoz M. , Tamargo M. C.
Published: February 2004

In this paper we show that a multilayer Zn‐Se‐Te system, grown by molecular beam epitaxy in such a way as to contain sub‐monolayer quantities of Te (so‐called delta‐doped ZnSe:Te), exhibits optical properties consistent with the presence of quantum size centers. We present results of excitation intensity and temperature dependent cw photoluminescence (PL) as well as time‐resolved PL measurements that explicitly show that the ensemble of type‐II quantum islands is formed in this system. Specifically, for the 2.46–2.51 eV PL we show that (1) its maximum shifts as a cube root of excitation intensity; (2) its full‐width‐at‐half‐maximum decreases with the temperature while its maximum undergoes a red shift in the temperature interval where the integrated PL intensity stays approximately constant; (3) the PL decay is non‐exponential at high excitation intensities and a single exponential decay at low excitation intensities; (4) the PL life‐time initially increases with temperature from of 86 ns at T = 15 K to 143 ns at T = 115 K before deceasing due to non‐radiative processes. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ferromagnetic phase in II–VI semiconductors controlled by carriers
Authors: Cibert J. , Boukari H. , Bertolini M. , Pacuski W. , Ferrand D. , Tatarenko S. , Kossacki P. , Gaj J.A.
Published: February 2004

One main interest of carrier‐induced ferromagnetism in diluted magnetic semiconductors, is that it allows an active and sometimes reversible control of the magnetic properties, simply by manipulating the carrier density or carrier distribution using the usual techniques of semiconductor technology. We shall give two examples realised in (Cd,Mn)Te quantum wells: (i) control of the magnitude of the magnetisation by changing the carrier density; (ii) control of the magnetic anisotropy by changing the light‐hole/heavy‐hole splitting by strain and confinement in a quantum well. We will also shortly discuss the peculiarities of the II–VI quantum wells with respect to systems based on other diluted magnetic semiconductors. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Magnetic resonant tunnelling diodes as voltage‐controlled spin selectors
Authors: Gould C. , Slobodskyy A. , Slobodskyy T. , Grabs P. , Becker C. R. , Schmidt G. , Molenkamp L. W.
Published: February 2004

We study the transport characteristics of all II–VI semiconductor resonant tunneling diodes made from the (Zn,Mn,Be)Se material system and with a quantum well of dilute magnetic material. The current–voltage characteristics of these devices exhibit a normal resonant tunneling diode resonance at zero external magnetic field. When a field is applied, the resulting spin splitting of the levels in the quantum well splits the transmission resonance into two separate peaks that move apart with increasing field. We interpret this as evidence that the tunneling is taking place through spin polarized levels. This device could therefore be viewed as a first step towards the demonstration of a voltage controlled spin filter. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
X‐ray, photo‐ESR and optical spectroscopy studies of ZnSxSe1−x solid solutions doped with Co
Authors: Surkova T. P. , Galakhov V. R. , Godlewski M. , Świątek K.
Published: February 2004

Experimental studies of X‐ray photoelectron and Co Lα X‐ray emission spectra of the ZnS:Co semiconductor were carried out. It was established that Co ions are in a Co2+ configuration and that the Co 3d impurity states are localized above the top of the valence band by 1.0 ± 0.2 eV. It was found that the covalency reduction factor for Co2+ defect changes through the alloy from 0.62 in ZnSe lattice to 0.59 in ZnS. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
CdSe‐based nanostructures: growth, properties, lasers
Authors: Ivanov S. V. , Toropov A. A. , Shubina T. V. , Sorokin S. V. , Kyutt R. N. , Sitnikova A. A. , Solnyshkov D. D. , Nekrutkina O. V.
Published: February 2004

Paper presents an overview of recent results on novel CdSe quantum dot (QD) fabrication techniques with the main impact on a stressor‐controlled self‐organization process resulting in nucleation of real CdSe QDs optically active up to room temperature, with a narrower size distribution, a higher density and a high Cd content. The main idea of the stressor‐controlled epitaxy is an intentional introduction of a super‐strained fractional monolayer of the much higher lattice‐mismatch compound – stressor – to create strong local stress fields on the growth surface which govern self‐assembling of the main QD material. The studies are performed on a type‐II non‐common atom CdSe/BeTe system, where CdTe (Δa/a ∼ +14%) and BeSe (Δa/a ∼ −10%) interface bonds play a role of intrinsic stressors. Both experimental data (growth, structural and optical characterization) and Monte Carlo simulation of the growth process are presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
InAs/Zn(Mn)Te/Cd(Mn)Se pseudomorphic quantum well structures for spintronic applications
Authors: Toropov A. A. , Terent'ev Ya. V. , Shubina T. V. , Sorokin S. V. , Sedova I. V. , Kaygorodov V. A. , Solov'ev V. A. , Mel'tser B. Ya. , Ivanov S. V. , Kop'ev P. S. , Buyanova I.A. , Chen W. M.
Published: February 2004

Zn(Mn)Te/Cd(Mn)Se quantum well heterostructures have been grown pseudomorphically on InAs(001) substrates using original InAs surface preparation techniques. The structures demonstrated bright PL in the 1‐1.3 eV range and the effect of giant Zeeman splitting in the external magnetic field. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Giant polarized optical properties in type‐II ZnTe/CdSe multiple quantum wells induced by interface chemical bonds
Authors: Chen Y. F. , Su W. S. , Ya M. H. , Chiu Y. S.
Published: February 2004

Photoluminescence (PL) and photoconductivity (PC) spectra were studied in type‐II ZnTe/CdSe multiple quantum wells. It was found that the PL spectrum exhibits a strong in‐plane polarization with respect to 〈011〉 axis with polarization degree up to 30%. The degree of polarization does not depend on the excitation intensity and temperature, which excludes extrinsic mechanisms related to the observed in‐plane anisotropy. The underlying mechanism of the observed polarization dependence of the optical properties was attributed to the inherent property of the orientation of chemical bonds in ZnTe/CdSe heterostructures with no common atom. In addition, the PC spectra also display a strong polarization dependence. We therefore point out that polarized PL and PC spectroscopies provide a unique way to probe the effects of the orientation of interface chemical bonds on semiconductor heterostructures. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
How to avoid non‐radiative escape of excitons from quantum dots?
Authors: Robin I. C. , André R. , Dang Le Si , Mariette H. , Tatarenko S. , Gérard J. M. , Kheng K. , Tinjod F. , Bartels M. , Lischka K. , Schikora D.
Published: February 2004

We investigated the transition between two different exciton recombination regime in quantum dots depending on temperature. When temperature is raised above a threshold value that we determine experimentally non‐radiative recombination channels, characterized by their activation energy, dominate over radiative recombination. The analyses of time resolved photoluminescence spectroscopy versus temperature shows that the maximum temperature for dominant radiative recombination scales linearly with the activation energy of the non‐radiative channels over a large range of values (10–60 meV) measured for various kind of II–VI‐based quantum dots: CdTe/ZnTe, CdTe/ZnMgTe, CdSe/ZnSe. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Magneto‐photoluminescence study on magnetic/non‐magnetic semiconductor coupled quantum dots
Authors: Lee S. , Shin D. Y. , Titova L. , Kutrowski M. , Dobrowolska M. , Furdyna J. K.
Published: February 2004

Magneto‐photoluminescence experiments were carried out on self‐assembled quantum dots (QDs) fabricated in the form of two adjacent QD layers separated by a thin barrier. A strong PL peak is observed in this double‐layer QD system, which is identified as the ground state transition. The PL peak exhibits a large Zeeman shift, characteristic of structures involving diluted magnetic semiconductors (DMSs) when an external magnetic field is applied. However, the dependence of the PL intensity on magnetic field shows an entirely unexpected behavior in this double‐layer QD structure. In contrast to typical behavior of many low‐dimensional DMS systems, where the application of a magnetic field B dramatically increases the PL intensity, in double‐layer structures described above we observe a striking decrease of the PL intensity as a function of B. This effect indicates carrier tunneling from CdSe to CdMnSe dots as the band edges of CdMnSe are Zeeman‐shifted, thus increasing the exciton probability in the DMS QDs. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Dynamical properties of atom‐like emissions from single localized states in ZnCdS ternary mesa‐shaped structures
Authors: Kumano H. , Hitaka Y. , Suemune I.
Published: February 2004

Dynamical properties of exciton localization into spatially isolated localized states with atom‐like density of states caused by alloy potential fluctuations were examined. Submicron‐sized mesa‐shaped structures were prepared to resolve the contribution of single localized states to the optical spectra, which makes it possible to observe atom‐like emission lines and to discuss the dynamical properties of exciton energy relaxations by employing micro‐time‐resolved PL measurements. It will be shown that in the samples with high exciton energy transfer efficiency, photogenerated excitons can relax into the spatially isolated localized centers with strong zero‐dimensional nature, which leads to the limited number of atom‐like emission lines. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
P‐type doping and devices based on ZnO
Authors: Look D. C. , Claflin B.
Published: February 2004

Both n‐type and p‐type ZnO will be required for development of homojunction light‐emitting diodes (LEDs) and laser diodes (LDs). It is easy to obtain strong n‐type ZnO, but very difficult to create consistent, reliable, high‐conductivity p‐type material. The most natural choice of an acceptor dopant is N, substituting for O, and indeed several groups have been able to obtain p‐type material by such doping. Surprisingly, however, other groups have also been successful with P and As, elements with much larger ionic radii than that of O. Although ZnO substrates are now available, most of the epitaxial p‐type layers so far have been grown on sapphire, or other poorly‐matched materials. The lowest p‐type resistivity obtained up to now is about 0.5 Ω‐cm, which should be sufficient for LED fabrication. In spite of the present availability of p‐type ZnO, very few homojunction LEDs have been reported so far, to our knowledge; however, several good heterojunction LEDs have been demonstrated, fabricated with p‐type layers composed of other materials. One such structure, with fairly strong 389‐nm emission at 300 K, involves n‐type ZnO and p‐type AlGaN, grown on an SiC substrate. Also, an N+‐ion implanted ZnO layer, deposited by chemical vapor deposition on Al2O3, exhibits 388‐nm emission at 300 K and could be economical to produce. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Contactless electroreflectance studies of II–VI nanostructures grown by molecular beam epitaxy
Authors: Muñoz Martín , Lu Hong , Guo Shiping , Zhou Xuecong , Tamargo Maria C. , Pollak Fred H. , Huang Y. S. , Trallero‐Giner C. , Rodríguez A. H.
Published: February 2004

The interband transitions of a single quantum well structure of Zn0.53Cd0.47Se/Zn0.27Cd0.23Mg0.50Se, lattice matched to InP, and of a capped CdSe quantum dot structure have been investigated using contactless electroreflectance. From a comparison of the quantum well optical transitions with those calculated using the envelope function approximation we determined the band offsets for this system. The electroreflectance spectrum of the quantum dot structure shows transitions originating from all the portions of the sample including the quantum dots and the wetting layer. Assuming a lens shape geometry and that the effective height‐to‐radius ratio observed in uncapped quantum dots is preserved, the size of the capped quantum dots was determined using the observed electroreflectance transitions, and the effective mass approximation. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
14% sputtered thin‐film solar cells based on CdTe
Authors: Compaan A. D. , Gupta A. , Drayton J. , Lee S‐H. , Wang S.
Published: February 2004

Polycrystalline II–VI semiconductor materials show great promise for thin‐film photovoltaic cells and modules. Large‐area deposition of these II–VI semiconductors such as CdTe is possible by a variety of methods but the use of a plasma‐based method such as magnetron sputtering can have significant advantages. Here we present recent results in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin‐film PV community addresses issues related to the challenges of fabricating high efficiency tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell with 14.0% efficiency at one sun for an air‐mass‐1.5 global spectrum. In addition, we have studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or recombination junction for alloyed II–VI‐based top cells in a tandem solar‐cell configuration. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Exchange induced Zeeman splitting in resonant tunnelling diodes
Authors: Estanislau J. F. , Bittencourt A. C. R. , Marques G. E.
Published: February 2004

We have studied effects of symmetry on spin dependent transport in CdTe‐Cd1−xMnxTe magnetic tunneling diodes. The tunneling and shot‐noise currents may give complementary information on charging processes of the resonant levels. Optimized sample design can explore the exchange induced Zeeman splitting to obtain efficient spin‐filters. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Near band edge emission of MBE grown ZnO epilayers: identification of donor impurities and O2 annealing effects
Authors: Morhain C. , Teisseire‐Doninelli M. , Vézian S. , Deparis C. , Lorenzini P. , Raymond F. , Guion J. , Neu G.
Published: February 2004

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the near band edge emission (NBE) spectra of unintentionally doped ZnO epilayers. It is shown that the different NBE transitions involve donor centres only. The lines can be divided into two groups. Those lying on the low energy side consist of recombinations of neutral‐donor bound exciton complexes, while those lying just below the A free excitonic emission correspond to donor‐to‐valence band transitions. One of the donors is identified as indium. The emission energy of the In‐donor bound exciton, the spectra of the bound exciton excited states, the In‐related D0h transition energy, and the electronic spectrum of the In‐donor are established. The binding energies of another three donor centres are evaluated. The occurrence of two of these centres is shown to depend on the stoechiometry, as revealed by annealing experiments in an O2 atmosphere. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Formation of ohmic contacts to p‐type ZnO
Authors: Kurimoto Makoto , Ashrafi A. B. M. Almamun , Ebihara Masato , Uesugi Katsuhiro , Kumano Hidekazu , Suemune Ikuo
Published: February 2004

Formation of ohmic contacts to p‐type zinc oxide (ZnO) was studied. The p‐type ZnO samples were grown by metalorganic molecular‐beam epitaxy with diethylzinc, deionized water vapor and monomethylhidrazine and were annealed under oxygen gas ambient at 650 °C or 700 °C for 20 min to activate doped nitrogen acceptors. Although the current‐voltage characteristics measured through gold p‐contacts were not perfectly ohmic, they showed perfectly ohmic properties after rapid thermal annealing (RTA) in the temperature range of 300 to 520 °C for 2 min. The ohmic contact resistivity was measured with the transmission‐line measurement method and it was decreased to approximately 1/64 with increasing the RTA temperature. The minimum contact resistivity of 3.15 × 10−3 Ω cm2 was observed with RTA at 520 °C for 2 min. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
ZnS‐based photonic crystal phosphors fabricated using atomic layer deposition
Authors: King J. S. , Neff C. W. , Blomquist S. , Forsythe E. , Morton D. , Summers C. J.
Published: February 2004

The infiltration by atomic layer deposition of three‐dimensional opal structures has been investigated as a means of fabricating photonic crystal phosphors. ZnS:Mn infiltrated and inverse opals have been demonstrated with filling fractions >95%. Characterization of these structures by scanning electron microscopy, specular reflectance, and photoluminescence is reported. Specular reflectance measurements confirm successful infiltration, and demonstrate modification of the electromagnetic density of states consistent with calculated photonic (111) pseudo band gaps. Photoluminescence measurements reveal modification of emission by the photonic crystal consistent with angular dependent specular reflectance measurements. These results reveal a flexible and convenient route for fabricating high performance photonic crystal structures and optical microcavities. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Epitaxial ZnO growth and p‐type doping with MOMBE
Authors: Suemune Ikuo , Ashrafi ABM. Almamun , Ebihara Masato , Kurimoto Makoto , Kumano Hidekazu , Seong Tae‐Yeon , Kim Bong‐Joong , Ok Young‐Woo
Published: February 2004

Control of ZnO and CdO crystalline structures and p‐type doping in ZnO were studied. Formation of zincblende ZnO layers as well as zincblende CdO/ZnO short‐period superlattices (SPSL) are demonstrated and bright luminescence was observed even at room temperature from the CdO/ZnO SPSL. p‐Type conductivity observed in N‐doped ZnO is studied and the relation to the photoluminescence peaks is discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Determination of the dielectric functions of MBE‐grown Zn1−xMgxTe II‐VI semiconductor alloys
Authors: Franz A. J. , Peiris F. C. , Liu X. , Bindley U. , Furdyna J. K.
Published: February 2004

We have explored the dielectric functions of ternary Zn1−xMgxTe thin films using a variable angle spectroscopic ellipsometer for samples between x = 0 and x = 0.52. We obtained values for the complex dielectric function for Zn1−xMgxTe in both the transparent and absorption regions by incorporating a threelayer model to simulate the experimental data. To this end, we also used the previously published relations of the dispersion of the indices of refraction (in the transparent region) of Zn1−xMgxTe measured using a combination of prism coupler and reflectivity. We have fitted the second derivatives of both the real and the imaginary parts of the dielectric function to obtain the critical point parameters corresponding to the higher order electronic transitions in the lattice. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Quantum Hall ferromagnetism in II–VI based alloys
Authors: Jaroszyński J. , Andrearczyk T. , Karczewski G. , Wróbel J. , Wojtowicz T. , Papis E. , Kamińska E. , Piotrowska A. , Popović Dragana , Dietl T.
Published: February 2004

The article reviews our recent studies on quantum Hall ferromagnetism (QHF) in diluted magnetic semiconductors. We carried out magnetoresistance studies on modulation‐doped, gated heterostructures of (Cd,Mn)Te/(Cd,Mg)Te:I.We put into evidence the formation of Ising quantum Hall ferromagnet with Curie temperature TC as high as 2 K. QHF is manifested by anomalous magnetoresistance maxima. Moreover, magnitude of these spikes depends dramatically on the history of the sample, shows hysteresis when either magnetic field or gate voltage are swept, stretched‐exponential time evolution characteristic of glassy systems, and strong Barkhausen noise. Our study suggests that these metastabilities stem from the slow dynamics of ferromagnetic domains. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
CdZnTe and CdTe materials for X‐ray and gamma ray radiation detector applications
Authors: Szeles Csaba
Published: February 2004

Good detection efficiency and high energy‐resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X‐ray and gamma‐ray detection applications such as medical and industrial imaging, industrial gauging and non‐destructive testing, security and monitoring, nuclear safeguards and non‐proliferation, and astrophysics. Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd1−xZnxTe‐based detector technology. Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd1−xZnxTe detector performance. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Self‐assembled quantum dot formation induced by surface energy change of a strained two‐dimensional layer
Authors: Tinjod Frank , Mariette Henri
Published: February 2004

To account for the occurrence (or not) of the Stranski‐Krastanow (SK) transition (two‐dimensional to 3D change of surface morphology) during the epitaxial growth of various lattice‐mismatched semiconductor systems, we present a simple equilibrium model taking into account not only the lattice mismatch, but also the dislocation formation energy and the surface energy. It demonstrates the importance of these parameters especially for II–VI systems such as CdTe/ZnTe and CdSe/ZnSe. For II–VIs indeed, as misfit dislocations are easier to form than in III–Vs (such as InAs/GaAs) or IV systems (Ge/Si), the 3D elastic transition is short‐circuited by the plastic one. Nevertheless, by lowering surface energy, telluride and selenide quantum dots can also be grown as predicted by our model and as evidenced experimentally by reflection high‐energy electron diffraction (RHEED), atomic force microscopy and optical measurements. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth and characterization of ultra‐thin quantum wells of II–VI semiconductors for optoelectronic applications
Authors: Hernández‐Calderón Isaac , García‐Rocha Miguel , Díaz‐Arencibia Pedro
Published: February 2004

Atomic layer epitaxy (ALE) was used to grow CdTe and CdSe quantum wells (QWs) with 1 to 4 monolayer thickness. With this ultra‐thin quantum wells (UTQWs) it is possible to cover the red‐blue spectral range. Low temperature photoluminescence spectroscopy reveals that most of these UTQWs present very intense and narrow emission and, depending on growth conditions, no thickness fluctuations are observed over the whole sample area. The heterostructures were grown at different temperatures between 260 and 290 °C. Both CdTe and CdSe UTQWs grown at the higher temperature presented blue‐shifted emission compared to those grown at lower temperatures. This effect is attributed to Cd losses by reevaporation and Cd substitution by Zn. This effect of the temperature can be employed for fine tuning of the UTQW emission. The critical thickness of CdSe/ZnSe grown by ALE is estimated to be 5 ML. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Selected area growth of II–VI nanostructures using shadow masks
Authors: Schallenberg T. , Schumacher C. , Brunner K. , Molenkamp L. W.
Published: February 2004

We investigate molecular beam epitaxy of II–VI materials through GaAs/AlGaAs epitaxial shadow masks on GaAs(001) substrate. Scanning electron and atomic force microscopy are employed to investigate the effects of surface diffusion, faceting, partial shadow (minimized in the experimental set‐up), and material deposition on the mask (closing of apertures) on the selected area growth of ZnSe and CdSe. The results illustrate that selected area growth of II–VI nanostructures is controlled by the growth geometry. In contrast to the case of III–V materials, we found no macroscopic features (>10 nm) which could be attributed to surface diffusion. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Spin–lattice relaxation in diluted magnetic (Cd,Mn)Se quantum dots
Authors: Puls J. , Hundt A. , Thomas H. , Henneberger F.
Published: February 2004

Spin–lattice relaxation (SLR) is studied in quantum well and quantum dot structures by photo‐carrier induced spin heating. For the SLR time constants no qualitative difference is found between quantum well and quantum dot structures. There is no hint for a specific zero‐dimensional behaviour. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical phonons and resonant Raman scattering in II–VI spheroidal quantum dots
Authors: Trallero‐Giner C.
Published: February 2004

First order and multiphonon resonant Raman scattering processes from confined and interface polar optical phonons in II–VI nanocrystallites are analyzed. Fröhlich interaction between excitons and optical phonons has been considered and general selection rules for the exciton–phonon matrix elements of the scattering processes in the case of spheroidal quantum dots are given. A comparison is made between the Raman selection rules and non‐linear hyper‐Raman selection rules for spherical dots. It is shown that the relative intensities between phonon overtones in II–VI spherical quantum dots can be correctly described in the framework of excitonic model and the optical polar vibrational continuum model satisfying both the mechanical and electrostatic matching conditions at the interface. Detailed discussions of the surface optical phonons are provided in the case of a spherical Quantum‐Dot/Quantum‐Well (QD/QW) heterostructure. Their frequency dependence on the geometry, material parameters, as well as the electron–phonon interaction Hamiltonian are also reported. In addition, the interface optical phonons and the corresponding electron–phonon interaction in nanostructures with prolate and oblate spheroidal geometries and their contribution to the first order scattering process have been studied. This surface‐optical phonon modes are strongly dependent on the nanocrystal geometry and new Raman selection rules are obtained given a plausible explanation for the observed low frequency shoulders present in the spectrum of several II–VI semiconductor nanostructures. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)