J. Nürnberger's research while affiliated with University of Wuerzburg and other places

Publications (42)

Article
Despite the successes reached with wide gap II–VI materials, there is much work to be done in improving these devices in order to increase the lifetime. Two important points in this context that have been investigated in Würzburg are an improvement of p-contacts, which are still not very reproducible, and a decrease of threshold current densities....
Article
Full-text available
A concept of mixed exciton-trion states is formulated theoretically and proved experimentally for II–VI semiconductor quantum wells with a two-dimensional electron gas. The concept considers the resonances of neutral excitons and charged excitons (trions) as mixed (with each other) via their interaction with free electrons. Reflectivity spectra of...
Article
A selective dry etching technique for II–VI semiconductors based on ZnSe has been developed by using thermally assisted electron cyclotron resonance etching with a gas mixture of Ar, Cl2, and BCl3. While the etching process is found to be almost nonselective between ZnSe and MgZnSSe at low (≪100 °C) and high (≫220 °C) substrate temperatures, a stro...
Article
Linear polarization of the nitrogen acceptor bound exciton I1N luminescence, which reflects the anisotropic strain relief of the ZnSe lattice, is exploited to monitor two independent relaxation processes in nitrogen-doped ZnSe films. An inhomogeneous vertical strain profile is identified by a maximum in-plane polarization when 800 nm of the sample...
Article
The oscillator strength of negatively charged excitons (trions) in ZnSe/(Zn,Mg)(S,Se) quantum-well structures with n-type modulation doping is studied by means of reflectivity as a function of electron concentration, temperature, and external magnetic fields. The trion oscillator strength is found to increase linearly with increasing electron conce...
Article
We demonstrate that the quaternary compound ZnMgSeTe can be grown by molecular beam epitaxy in reasonable quality. For layers with energy gaps as high as 3.1 eV, nitrogen doping leads to free hole concentrations around 1018 cm−3. In combination with n-ZnMgCdSe, this material allows the fabrication of II–VI diodes lattice matched to InP substrate. L...
Article
We report magneto-optical studies of ZnSe/(Zn,Mg)(S,Se) quantum wells with n-type modulation doping. Polarized photoluminescence and spin-flip Raman scattering spectroscopy is used to investigate the energy and spin structure of neutral and negatively charged excitons. The binding energy of charged excitons changes very little in magnetic fields up...
Article
We present an analysis of the dependence of the line shape of four-wave-mixing (FWM) signals on the sample geometry and the excitation conditions based on ZnSe single quantum wells with varying width and barrier composition. We analyze the geometry dependence of the biexciton binding energy and observe a strong influence of reabsorption and of the...
Article
We present results of optical degradation experiments on II–VI laser diodes. The illumination is done with a microfocus to allow the degradation of stacking-fault-free regions. A degradation is only observed if the excited region (in our case the waveguide) is doped with nitrogen. Parallel to the decrease of the quantum-well luminescence, we observ...
Article
The k-resolved band structure of an epitaxially grown ZnSe(100) layer was investigated by angle resolved ultraviolet photoelectron spectroscopy combined with ab initio band structure calculations. The ZnSe layer 4μm thick was grown on a GaAs(100) substrate by molecular beam epitaxy and was transported to the German synchrotron facility BESSY under...
Article
Available from http://www3.interscience.wiley.com/cgi- bin/abstract/72507416/START
Article
We report on reflectivity studies of negatively charged excitons (trions) and exciton states in ZnSe/(Zn,Mg)(S,Se) quantum wells with a two-dimensional electron gas of low density (ne⩽1.2×1011 cm−2). The radiative damping (i.e. oscillator strength) of the trion states grows linearly with increasing electron concentration. The exciton radiative damp...
Article
We use density functional theory to examine the stability of defects in p-ZnSe that are formed by a transition of the nitrogen acceptor to the interstitial site. We find that the threefold positive complex of the interstitial nitrogen with the remaining vacancy is considerably more stable than the nitrogen acceptor. The formation of the complex is,...
Article
Full-text available
We report on magneto-optical studies of ZnSe/(Zn,Mg)(S,Se) quantum wells with n-type and p-type modulation doping. Negatively and positively charged excitons related to the heavy-hole exciton states are found and identified by their polarization properties. Negatively charged excitons formed with light-hole exciton states are observed. Their bindin...
Article
This contribution reports about the first in situ lateral structured II–VI semiconductors grown with molecular beam epitaxy into Al50Ga50As–GaAs shadow masks. The lateral control is achieved by tilting the sample manipulator with respect to the molecular beam emanated by the effusion cells. Thus wire structures can be grown whose width depends on t...
Article
ZnSe-(Zn,Mg)(S,Se) and MgSe-Zn(S,Se) superlattices are presented here. The first one, ZnSe-(Zn,Mg)(S,Se) is a type I structure, while the second one, MgSe-Zn(S,Se) is of type II. Both kinds of superlattices can be grown with excellent optical and structural quality which is confirmed by high resolution X-ray diffraction (HRXRD) and photoluminescenc...
Article
We present results of optical degradation experiments on blue-green ZnSe-based diode laser structures. A micro-focused wavelength selectable laser is used for degradation and the luminescence from the degrading region is simultaneously recorded. The degradation speed at a given optical power density depends strongly on the doping of the structure a...
Article
A systematic analysis of excitonic and biexcitonic contributions to four-wave mixing in dependence on the polarization state and on the intensity of the exciting beams is presented for ZnSe/Zn1—xMgxSySe1—iiy quantum wells. The relative signals of excitons and biexcitons for different polarization configurations are discussed in terms of a microscop...
Article
We show that the formation of ohmic contacts to p-ZnSe does not mainly depend on the exact design of the applied ZnSe/ZnTe grading, but rather on the temperature during contact growth and the thickness of the ZnTe cap layer on top of the grading. These facts demonstrate that a diffusion process dominates the contact formation. From the activation e...
Article
The energy gap of zinc-blende epitaxial layers grown by molecular beam epitaxy has been determined over a wide range of composition using optical transmission and reflection and an empirical formula for the energy gap at room temperature is also given. In addition in situ spectroscopic ellipsometry was used in order to determine the temperature dep...
Article
We have fabricated II–VI wide gap hetero PIN photodiodes made of ZnMgSSe with excellent structural and interface quality and with a high external quantum efficiency of about 60%, which is close to the theoretical limit. The internal quantum efficiency reaches peak values of more than 80%. The onset of the sensitivity is very sharp, leading to a qua...
Article
Heterostructures with ZnSe quantum wells and different barrier materials (Zn1 − xMgxS1 − ySey and Zn1 − xBexSe) are investigated by means of photoluminescence and reflectivity in external magnetic fields up to 20 T. Up to four exciton excited states are well-resolved in the photoluminescence excitation spectra of undoped structures. Model calculati...
Article
We report on the anisotropic polarization of the luminescence observed in ZnSe-based heterostructure lasers. It has been found that light originating from the radiative recombination of electrons and holes confined in the ZnCdSe quantum well of the laser is linearly polarized collinearly to one 〈110〉 crystallographic direction after having passed t...
Article
A low damage in situ compatible etching technology for the patterning of wide bandgap II–VI materials has been developed. Thermally assisted ECR-etching in a UHV chamber is used for the pattern transfer into the semiconductor heterostructure while in situ and ex situ photoluminescence (PL) spectroscopy is performed to analyze the etched structures....
Article
Lateral patterning techniques have been applied to control the transversal and longitudinal mode structure in ZnSe-based laser diodes. For ridge wave-guide lasers with a stripe width Lx of less than 5 μm, predominantly lateral single-mode emission was observed in the near-field and the far-field emission patterns. For the ratio of the vertical and...
Article
ZnSe p-i-n-type photodiodes were grown on GaAs substrates using molecular beam epitaxy technique. ZnCl2 was used for n-doping and plasma-excited N2 for p-doping. The p-side contact consists of 200 nm Au in situ deposited on a 20 nm highly doped ZnTe layer on top of the p-doped ZnSe forming a well-defined Schottky barrier, as can be confirmed by cur...
Article
We present studies of electrical properties, electroluminescence (EL) and degradation behaviour of laser diodes grown on GaAs, terminated with Te (Te: GaAs). Compared to conventional lasers (grown on Zn-treated GaAs), the lasers grown on Te : GaAs show a comparable stacking fault density, but higher spontaneous emission and very low threshold curre...
Article
Implantation-induced disordering in ZnCdSe-based quantum wells and superlattices is investigated and used to generate lateral index-guided laser structures. The structures are implanted with Nitrogen at doses of 1 × 10154 × 1015 cm−2 at energies ranging between 110 and 230 keV and subsequently annealed at temperatures up to 460°C in Zn-atmosphere....
Article
The start of relaxation of ZnSe epitaxial layers grown on GaAs(0 0 1) substrates by the formation of dislocations will be discussed in this article. The minimum dislocation density, which is detectable with high-resolution X-ray diffraction ω − 2Θ scans, is limited by the minimum measurable parallel lattice misfit. According to this criterion nearl...
Article
Longitudinal single mode emission from ZnSe-based laser diodes with first order distributed Bragg reflectors has been achieved. The emission energy could be varied in the green spectral range over more than 80 meV by changing the grating period from 97.7 to 102.2 nm. The variation of the lasing threshold current in this range is explained as a cons...
Article
A low damage dry etch technology suitable for in situ processing was developed for the fabrication of ZnSe-based nanostructures. Thermally assisted electron cyclotron resonance etching combines plasma etching at low ion energies with process temperatures between 80°C and 210°C. Due to a variation of the process parameters, i.e., plasma power and sa...
Article
The doping dependent intermixing of ZnCdSe/ZnSe superlattices was studied by secondary ion mass spectroscopy. The chlorine or nitrogen doped and undoped structures were grown by molecular beam epitaxy. Heat treatment was performed in the temperature range of 300 to 550 °C under different conditions, namely Zn or N atmosphere, vacuum and protected b...
Article
We show that a drastic improvement of the current voltage characteristics of a ZnSe diode can be achieved by lowering the growth temperature of a ZnSe/ZnTe multi-quantum well p contact. A similar trend is observed when the thickness of the ZnTe cap layer on top of the multi-quantum well is reduced. Both observations show that the suppression of int...
Article
Beryllium-containing ZnSe-based compound semiconductors introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures. An overview of the advantages of beryllium chalcogenides is given here. A variety of BeTe-ZnSe and BeMgZnSe-ZnSe structures has been fabricated by molecular beam epitaxy, and their structural...
Article
Compensation in ZnSe:N was investigated. The total nitrogen concentration in the samples was determined by two different techniques: secondary ion mass spectroscopy and nuclear reaction 15N(p, αγ)12C analysis. The net free hole concentration NA − ND was correlated with these results. A ratio of up to 50% was observed. Furthermore, we studied the ex...
Article
Structural, optical and electrical properties of green and blue emitting room temperature laser diodes grown by molecular beam epitaxy are presented. The influence of the growth start at the heterointerface is discussed based on cross sectional and plan view transmission electron microscopy studies. GaAs buffer layers are necessary for reproducible...
Article
The properties of ZnSe and ZnSxSe1−x epilayers grown by molecular beam epitaxy deposited either directly on GaAs substrates or on GaAs buffer layers are compared with each other. The superior structural quality of the latter one is confirmed by reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD) and transmission electron mi...
Article
Growth and optical properties of MBE grown ZnSe:N are investigated. Doping levels between NA – ND = 1 × 1015 to 1 × 1018 cm−3 are achieved by using either a rf or ECR plasma source for nitrogen activation. The different behaviour of the photoluminescence for both doping methods is discussed. It is shown that during the operation of the rf plasma in...
Article
Low resistance ohmic contacts to p-ZnSe involving the use of HgSe are studied in detail. HgSe does not form an ohmic contact with p-ZnSe because of a residual valence band offset at the heterojunction of 0.5 to 0.6 eV. Annealing of HgSe/ZnSe:N does not result in an ohmic contact because a large miscibility gap in Hg1−xZnxSe prevents a HgZn interdif...

Citations

... We obtain τ 1 (0) = 1/(k 21 + k 23 ) = 5.3 ns, τ 2 (∞) = 1/(k 23 + k 31 ) = 14.5 ns, a(∞) = k 23 /k 31 = 6.4, and the corresponding lifetimes are summarised in table I. Remarkably, the value for τ 1 agrees well with the PL decay time of 6.1 ns observed in time-resolved experiments 23 and remarkably longer than that of the band-to-band transition in semiconductor nanostructures 27 . The excitation rate of V Si is proportional to the laser power density k 12 = σW , where the absorption cross section σ can be calculated from the saturation behaviour of Fig. 2(d) as σ = (hν/T W 0 )(k 23 k 31 + k 21 k 31 )/(k 23 + k 31 ). ...
... It is mainly due to that these compounds present covalent bonding [1] which is unique among II-IV compounds [2]. By combining with the other II-VI semiconductors, beryllium chalcogenides can exhibit direct band gap and represent the most appealing materials for improving the laser diodes [2][3][4][5]. ...
... However, a breakthrough for a reliable technology on this material is hampered by the lack of long-term stability of these devices [1]. Apart from the reduction of stacking faults density, their degradation has been related to two factors: firstly, problems connected with the nitrogen doping used to obtain p-type conductivity (nitrogen related deep complex centers act as efficient nonradiative centers during operation) [2,3] and secondly, the instability of the quantum well (QW) during operation, which is regarded to be the main obstacle for the application of II-VI materials for light emitters. The gradual decrease of the output intensity in the ZnSe-based LD during operation can be related to an occurrence of dark spots and dark line defects [4], but microstructural investigations of the defect formation mechanisms are rare and not really conclusive [5,6]. ...
... Particularly, we have observed that the X − polarization degree is slightly lower than the X polarization degree. If we suppose that our system is an isolated QW under thermal equilibrium, we should expect some difference in the polarization degree of the PL emission from trions and excitons due to their different g-factors and lifetimes [23][24][25]. In our case, the situation is even more complex, as our system has some particularities if we compare to an isolated QW. ...
... Zinc selenide (ZnSe) is known to be a promising material amongst II-VI semiconductors for their useful applications in the blue-violet and Ultra-violet photodetectors due to its wide direct band gap and almost constant responsivity in the blue-UV region. While the previous attempts to use single crystal ZnSe in different layered structures, such as, the metal-semiconductor-metal structures [1][2][3][4][5][6], Schottky contacts [3,[7][8][9][10][11] or p-i-n diodes [3,10,[12][13][14] have revealed photo-response to vary widely from one structure to other, it is a matter of concern if the UV sensitivity of comparable order can be attained with polycrystalline form of ZnSe. It is well known that the optical and electrical properties of polycrystalline materials differ significantly compared to their single crystal counterparts due to the presence of grain boundaries typical for material in the polycrystalline form. ...
... Semiconducting materials remain of great interest due to their central role in modern electronics [1][2][3][4][5][6][7]. In this era of designing optimal materials, it has become essential to estimate quickly and accurately their bandgaps. ...
... However, despite numerous efforts during the last decade towards optimisation of ZnSebased quantum well (QW) laser diodes, their lifetime is still limited to 400 h in a CW regime [1]. This is mainly due to the metastable nature of N-acceptors being the main origin of so-called 'slow'degradation of the laser diodes [2,3]. The alternative way to obtain green laser emission is an integral III-N=II-VI blue-green laser converter, in which the blue emission of a III-N-based laser is used for optical pumping of a CdSe=ZnSe quantum dot (QD) laser. ...
... Detailed information about samples no. 1 and no. 2 and the typical optical and magneto-optical properties of ZnSe-based QWs can be found in Refs. [35][36][37][38][39]. ...
... In the simplest picture, a trion is a three-particle complex consisting of a carrier from the Fermi sea bound to the photoexcited e-h pair. Trions appear as an additional optical resonance below the neutral exciton, and the energy difference between the two provides a measure of the binding energy between the exciton and the resident carrier: ∼ 1-5 meV in semiconductors such as GaAs or ZnSe [16][17][18], and ∼ 20-30 meV in monolayer (ML) semiconductors such as MoS 2 or WSe 2 [19]. ...
... II-VI mix6ed crystals found several applications in modern optoelectronics, including the construction of visible radiation sources [1], photodetection [2], green laser diodes [3], electro-optic modulators [4], solar cells [5], and infrared devices [6]. Essential properties of ternary and quaternary compounds are changes in the energy band gap and lattice constant values with a bit of compositional modification. ...