physica status solidi (RRL) - Rapid Research Letters

Ge–Sb–Te (GST) alloys lying along the GeTe–Sb₂Te₃ pseudo-binary tie line have proven to be among the most auspicious classes of materials for storage applications. Their success is related to the rapid and reversible phase-change from the crystalline to the amorphous state, whose strongly different optical and electrical properties can be used to encode information. The main technique used to produce phase-change material (PCM) thin films, both in academia and in industry, is physical vapor deposition (PVD). The layers fabricated by PVD are polycrystalline in the crystalline state. In their Rapid Research Letter on pp. 415–417, Peter Rodenbach et al. demonstrate the successful molecular beam epitaxy (MBE) of PCM thin films in the quasi-binary GeTe–Sb₂Te₃ system on slightly (GaSb) as well as largely (Si) lattice-mismatched substrates with (111) orientation. MBE growth is anticipated to allow the control of the alloy composition at the atomic level of the different chemical species. The perfect spatial order of epitaxial layers opens new horizons for investigation of the unique properties of PCMs, that may lead to the development of conceptually new memory devices.

Flexible GaN-based light-emitting diodes (LEDs) on polyethylene terephthalate (PET) substrates are demonstrated by Younghun Jung et al. in their Letter on pp. 421–423. The process uses commercial LEDs on patterned sapphire substrates, laser lift-off (LLO), wet etching for additional surface roughening, and mounting of the freestanding LED on a PET substrate. Electrical and optical properties from the free-standing LLO-LEDs mounted on the flexible PET substrates were characterized. This process, illustrated on the back cover, is highly practical and can be easily implemented in a manufacturing process to fabricate large-scale flexible GaN-based LEDs.

Rodenbach, Calarco and co-workers report the successful epitaxial growth of the phase-change material Ge₂Sb₂Te₅ (GST) on GaSb(001), GaSb(111), and Si(111) using molecular beam epitaxy. The structural properties are investigated by reflection high energy electron diffraction, synchrotron X-ray diffraction and transmission electron microscopy. The films exhibit a superior quality if grown on (111) surfaces, independent of the lattice mismatch. This is attributed to the tendency of this material to form layered structures as well as to the nature of distortion in the metastable cubic phase.

This Letter reports on an Auger leakage model as an intrinsic side effect of the Auger process. It has a significant impact on the internal quantum efficiency versus current characteristics of wide band gap III-nitride quantum-well light emitting diodes (LEDs). Compared to a standard Auger model, Auger leakage reduces the Auger coefficient needed to explain experimental data by 50%, with all other parameters kept constant.

GaN-based flexible light-emitting diodes (LEDs) on polyethylene terephthalate (PET) substrates are demonstrated. The process uses commercial LEDs on patterned sapphire substrates, laser lift-off, wet etching for additional surface roughening, and mounting of the freestanding LED on a PET substrate. Bright electroluminescence was observed under strain.

The present study shows for the first time that coating of a Si-nanoparticle film on top of a dye-sensitized solar cell serves as an ultraviolet to visible light convertor as well as an anti-reflector, resulting in improved cell efficiency. The coating extends the spectral region along with the homogeneous irradiation.

A new graphane allotrope with a 4up/2down configuration is proposed and named as tricycle, which can be considered as the combination of the 3up/3down chair and stirrup graphanes. Tricycle graphane possesses remarkable stability comparable to the most stable chair indicating its high probability to coexist with chair graphane in the process of graphene hydrogenation.

A nanocone-based highly transparent and flexible superhydrophobic surface with water contact angle greater than 150º has been fabricated at room temperature by applying the ion irradiation method. This novel one-step strategy might be very promising to fabricate transparent and flexible superhydrophobic surfaces at large scale for potential applications in various fields.

An enhanced interlayer mass transport by adopting an interval pulsed-laser deposition (PLD) process results in a persistent layer-by-layer growth as demonstrated for homoepitaxial ZnO thin films. The surface roughness is considerably reduced for the interval PLD approach as compared with a conventional PLD. In this Letter, Zippel and coworkers compare carefully the properties of homoepitaxial ZnO thin films deposited under identical growth conditions by interval and by conventional PLD.

The piezoelectric coefficient d₃₃ of ZnO:Mn films is a key factor determining the performance of ZnO:Mn/Si surface acoustic wave (SAW) devices, and the SAW properties can be improved by doping with an appropriate Mn concentration. The absolute value of the insertion loss is inversely proportional to d₃₃, while the electromechanical coupling coefficient and bandwidth are both proportional to d₃₃. A Mn–ZnO/Si multilayer structure with large d₃₃ is promising for high frequency, wide-band and low-loss SAW applications.

Trimethylaluminum (TMA) pretreatment prior to atomic layer deposition of HfO₂ on a Ge substrate is introduced. Secondary ion mass spectroscopy reveals less Ge diffusion into the HfO₂ film after TMA pretreatment because the germanium native oxide is converted to aluminum oxide, resulting in a capacitance–voltage hysteresis reduction and improved device reliability.

Today, ultraviolet (UV) photodetectors are drawing a considerable attention due to their importance in many applications including space communication, environmental monitoring, military and civil applications. In this Letter, the fabrication and characterization of highly sensitive Schottky-type UV photodetectors based on Au/TiO₂ nanorods are demonstrated. The fabricated devices function as highly sensitive UV photodetectors with a peak responsivity of 134.8 A/W at a wavelength of 350 nm under reverse bias of 3 V.

Pilkyung Moon et al. investigated the effects of post-growth thermal annealing on the optical properties of quantum dots by using (i) the etched micro-mesa to identify the emission signal from the dots, (ii) the superlattice with the same material composition as the dot to make a quantitative estimation of the diffusion length, and (iii) a numerical simulation to provide the diffusion length dependence of the emission energies of the dots.

Wang et al. report that the value and sign of the exchange bias field can be tuned effectively through zero field cooling from different remanent states above the blocking temperature in Ni₅₀Mn₃₆Sb₁₄ alloy, in addition to the field cooling method. This gives the opportunity to tune the exchange bias even after device fabrication and may be interesting for novel types of magnetoelectronic devices.

In this Letter, the phase diagram of the ternary (Bi_0.5Na_0.5)TiO₃–(Bi_0.5K_0.5)TiO₃–(K_0.5Na_0.5)NbO₃ (BNT–BKT–KNN) system is drawn, and the switching characteristics of the morphotropic phase boundary (MPB) are investigated. An evident MPB switching behavior is found with the addition of KNN as a replacement for BNT–BKT. Near the MPB region, excellent electrostriction with good thermostability comparable with that of traditional Pb-based electrostrictors is achieved.

As shown in this Letter, Park et al. investigated the characteristics of a simple cross-point resistive-switching random access memory (RRAM) array using a self-formed Schottky barrier fabricated without any additional fabrication steps. The device provides an obvious rectification in the low-resistence state (LRS), so the crosstalk phenomenon can be alleviated without serially connecting a diode. On the basis of the characterization results, the authors demonstrate set, reset, and read operation for high cell efficiency and high density memory applications.