Jonas Lähnemann

Continuous-flux MOVPE growth of position-controlled N-face GaN nanorods and embedded InGaN quantum wells

We demonstrate the fabrication of N-face GaN nanorods by metal organic vapour phase epitaxy (MOVPE), using continuous-flux conditions. This is in contrast to other approaches reported so far, which have been based on growth modes far off the conventional growth regimes. For position control of nanorods an SiO(2) masking layer with a dense hole pattern on a c-plane sapphire substrate was used. Nanorods with InGaN/GaN heterostructures have been grown catalyst-free. High growth rates up to 25 microm h(-1) were observed and a well-adjusted carrier gas mixture between hydrogen and nitrogen enabled homogeneous nanorod diameters down to 220 nm with aspect ratios of approximately 8:1. The structural quality and defect progression within nanorods were determined by transmission electron microscopy (TEM). Different emission energies for InGaN quantum wells (QWs) could be assigned to different side facets by room temperature cathodoluminescence (CL) measurements.

Luminescence of GaAs nanowires consisting of wurtzite and zinc-blende segments

GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E(g,ZB) are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E(g,WZ)-E(g,ZB).

Direct experimental determination of the spontaneous polarization of GaN

We present a universal approach for determining the spontaneous polarization Psp of a wurtzite semiconductor from the emission energies of excitons bound to the different types of stacking faults in these crystals. Employing micro-photoluminescence and cathodoluminescence spectroscopy, we observe emission lines from the intrinsic and extrinsic stacking faults in strain-free GaN micro-crystals. By treating the polarization sheet charges associated with these stacking faults as a plate capacitor, Psp can be obtained from the observed transition energies with no additional assumptions. Self-consistent Poisson-Schroedinger calculations, aided by the microscopic electrostatic potential computed using density-functional theory, lead to nearly identical values for Psp. Our recommended value for Psp of GaN is -0.022+/-0.007 C/m^{2}.

Current path in light emitting diodes based on nanowire ensembles

Light emitting diodes (LEDs) have been fabricated using ensembles of free-standing (In, Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron-beam-induced current analysis, cathodoluminescence as well as biased μ-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In, Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect.

Luminescence associated with stacking faults in GaN

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should also be transferable to other wurtzite semiconductors.

Photoelectrochemical Properties of (In,Ga)N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy

We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)N NWs exhibited an anodic photocurrent, however, no O2 but only N2 evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H2. After photodeposition of Pt on such NWs, the photocurrent density was significantly enhanced to 5 mA/cm(2) at a potential of -0.5 V/NHE under visible light irradiation of ∼40 mW/cm(2). Also, incident photon-to-current conversion efficiencies of around 40% were obtained at -0.45 V/NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p-(In,Ga)N NWs are a promising basis for solar hydrogen production.

Coexistence of quantum-confined Stark effect and localized states in an (In,Ga)N/GaN nanowire heterostructure

We analyze the emission of single GaN nanowires with (In,Ga)N insertions using both micro-photoluminescence and cathodoluminescence spectroscopy. The emission spectra are dominated by a green luminescence band that is strongly blueshifted with increasing excitation density. In conjunction with finite-element simulations of the structure to obtain the piezoelectric polarization, these results demonstrate that our (In,Ga)N/GaN nanowire heterostructures are subject to the quantum-confined Stark effect. Additional sharp peaks in the spectra, which do not shift with excitation density, are attributed to emission from localized states created by compositional fluctuations in the ternary (In,Ga)N alloy.

Stacking faults as quantum wells in nanowires: Density of states, oscillator strength, and radiative efficiency

We investigate the nature of excitons bound to I1 basal-plane stacking faults [(I1,X)] in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i.e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the (I1,X) and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the (I1,X) recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of 210 cm s −1 considering the nanowires mean diameter of 50 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than 6 × 10 16 cm −3 . As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state.

Spatially resolved investigation of strain and composition variations in (In,Ga)N/GaN epilayers

The strain state and composition of a 400 nm thick (In,Ga)N layer grown by metal-organic chemical vapor deposition on a GaN template are investigated by spatially integrated x-ray diffraction and cathodoluminescence (CL) spectroscopy as well as by spatially resolved CL and energy dispersive x-ray analysis. The CL investigations confirm a process of strain relaxation accompanied by an increasing indium content toward the surface of the (In,Ga)N layer, which is known as the compositional pulling effect. Moreover, we identify the strained bottom, unstrained top, and gradually relaxed intermediate region of the (In,Ga)N layer. In addition to an increase of the indium content along the growth direction, the strain relaxation leads to an enhancement of the lateral variations of the indium distribution toward the surface.

Correlation between In content and emission wavelength of InxGa1−xN/GaN nanowire heterostructures

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.