S. L. Gu

Electroluminescent and transport mechanisms of n-ZnO∕p-Si heterojunctions

The distinct visible electroluminescence (EL) at room temperature has been realized based on n-ZnO∕p-Si heterojunction. The EL peak energy coincided well with the deep-level photoluminescence of ZnO, suggesting that the EL emission was originated from the radiative recombination via deep-level defects in n-ZnO layers. The transport mechanisms of the diodes have been discussed with the characteristics of current-voltage (I-V) and light-output–voltage (L-V), in terms of the energy band diagram of ZnO∕Si heterojunction. The tunneling mechanism via deep-level states was the main conduction process at low forward bias, while space-charge-limited current conduction dominated the carrier transport at higher bias. Light-output–current (L-I) characteristic of the diode followed a power law such as L∼Im, which showed a superlinear behavior at low injection current and became almost linear due to the saturation of nonradiative recombination centers at high current level.

Blue-yellow ZnO homostructural light-emitting diode realized by metalorganic chemical vapor deposition technique

We report on the realization of ZnO homojunction light-emitting diodes (LEDs) fabricated by metalorganic chemical vapor deposition on (0001) ZnO bulk substrate. The p-type ZnO epilayer was formed by nitrogen incorporation using N2O gas as oxidizing and doping sources. Distinct electroluminescence (EL) emissions in the blue and yellow regions were observed at room temperature by the naked eye under forward bias. The EL peak energy coincided with the photoluminescence peak energy of the ZnO epilayer, suggesting that the EL emissions emerge from the ZnO epilayer. In addition, the current-voltage and light output-voltage characteristics of ZnO homojunction LEDs have also been studied.

Fermi-level band filling and band-gap renormalization in Ga-doped ZnO

The fundamental optical properties of Ga-doped ZnO films grown by metalorganic chemical vapor deposition were investigated by room-temperature transmittance and photoluminescence (PL) spectroscopy. The Burstein–Moss (BM) shift of the absorption edge energy is observed at the carrier concentration up to 2.47×1019cm−3. The absorption edges are fitted to a comprehensive model based on the electronic energy-band structure near critical points plus relevant discrete and continuum excitonic effects, taking account of the Fermi-level filling factor. The theoretical calculation for BM effect is in good agreement with the experimental facts, considering the nonparabolic nature of conduction-band and band-gap renormalization (BGR) effects. Meanwhile, the monotonic redshift of the near-band-gap emission detected by PL measurements has also been observed with increasing free-carrier concentration, which is attributed to the BGR effects, and can be fitted by an n1∕3 power law with a BGR coefficient of 1.3×10−5meVcm.

Raman study of lattice dynamic behaviors in phosphorus-doped ZnO films

Phosphorus-induced lattice dynamic behaviors in ZnO:P epilayers grown by the metalorganic chemical vapor deposition technique have been studied using the Raman scattering method. Additional modes around 504, 520, 655, and 866cm−1 are attributed to the disorder-activated modes due to the breakdown of translational symmetry by P doping, well supported by the reported ab initio calculations of lattice dynamics in w-ZnO. Two modes around 364 and 478cm−1 are assigned to the local vibrational modes of Zn–P and P–O pairs, respectively. The correlation of transport and vibrational properties demonstrates the complex doping mechanism and the amphoteric nature of P dopant in ZnO. In addition, the redshift of 2 longitudinal optical multiphonon around 1154cm−1 is possibly originated from the variation of short-range forces in ZnO uniaxial lattice caused by P incorporation.

Production of high-quality ZnO films by the two-step annealing method

In this study, a two-step annealing method is advanced to produce high-quality ZnO films with excellent structural, electrical, and optical properties. The effects of oxygen and nitrogen annealing on the properties of undoped ZnO films are reversible to each other and are attributed to the creation and annihilation of extrinsic trap states of antisite oxygen OZn and oxygen vacancies VO, which result from the chemisorption and desorption of oxygen, respectively. Moreover, annealing in nitrogen causes slight nitrogen incorporation, subsequently increasing the resistivity and inducing compressive stress in the film. The key to this two-step method is to keep the chemisorption and desorption of oxygen in equilibrium. Due to the similarity of annealing ambient with the growth condition, this process can be transplanted and employed in the in situ preparation of high-quality ZnO epilayers.

Correlation between carrier recombination and p-type doping in P monodoped and In–P codoped ZnO epilayers

The carrier recombination processes in p-type ZnO epilayers with P monodoping and In–P codoping have been studied by temperature-dependent photoluminescence spectroscopy. Good correlations were observed between carrier recombination and acceptor and donor energy levels. The exciton transition feature of acceptor-bound excitons (3.350eV), the free electron-acceptor emission (3.315eV), and the donor-acceptor-pair emission (3.246eV) exhibited different carrier recombination associated various defect complexes. The origins of two broad emissions at ∼2.99 and ∼2.89eV were found to be due to different photoelectron radiative transitions associated with deep level acceptors (isolated Zn vacancies). The acceptor-bound energies for P monodoped and In–P codoped epilayers ∼195 and ∼127meV, respectively. The small binding energy is helpful for acceptor ionization at room temperature, resulting in a high hole concentration in the codoped epilayer.

Studies of metal–ferroelectric–GaN structures

A GaN-based metal–insulator–semiconductor (MIS) structure has been fabricated by using ferroelectric Pb(Zr0.53Ti0.47)O3 instead of conventional oxides as insulator gate. Because of the polarization field provided by ferroelectric and the high dielectric constant of ferroelectric insulator, the capacitance–voltage characteristics of GaN-based metal–ferroelectric–semiconductor (MFS) structures are markedly improved compared to those of other previously studied GaN MIS structures. The GaN active layer in MFS structures can reach inversion just under the bias of smaller than 5 V, which is the generally applied voltage used in semiconductor-based integrated circuits. The surface carrier concentration of the GaN layer in the MFS structure is decreased by one order compared with the background carrier concentration. The GaN MFS structures look promising for the practical application of GaN-based field effect transistors.

Nonpolar m-plane thin film GaN and InGaN∕GaN light-emitting diodes on LiAlO2(100) substrates

The nonpolar m-plane (11¯00) thin film GaN and InGaN∕GaN light-emitting diodes (LEDs) grown by metal-organic chemical vapor deposition on LiAlO2 (100) substrates are reported. The LEDs emit green light with output power of 80μW under a direct current of 20mA for a 400×400μm2 device. The current versus voltage (I-V) characteristic of the diode shows soft rectifying properties caused by defects and impurities in the p-n junction. The electroluminescence peak wavelength dependence on injection current, for currents in excess of 20mA, saturates at 515–516nm. This proves the absence of polarization fields in the active region present in c-plane structures. The light output intensity versus current (L-I) characteristic of the diode exhibits a superlinear relation at low injection current caused by nonradiative centers providing a shunt path and a linear light emission zone at high current level when these centers are saturated.

A van der Waals pn heterojunction with organic/inorganic semiconductors

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/optical properties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate the hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C8-BTBT) and n-type MoS2. We find that few-layer C8-BTBT molecular crystals can be grown on monolayer MoS2 by vdW epitaxy, with pristine interface and controllable thickness down to monolayer. The operation of the C8-BTBT/MoS2 vertical heterojunction devices is highly tunable by bias and gate voltages between three different regimes: interfacial recombination, tunneling, and blocking. The pn junction shows diode-like behavior with rectifying ratio up to 105 at the room temperature. Our devices also exhibit photovoltaic responses with a power conversion efficiency of 0.31% and a photoresponsivity of 22u2009mA/W. With wide material combinations, such hybrid 2D structures will offer possibilities for opto-electronic devices that are not possible from individual constituents.

Influence of doping on the two-dimensional electron gas distribution in AlGaN/GaN heterostructure transistors

A study of two-dimensional electron gas distribution in AlGaN/GaN heterostructure field effect transistors is performed by solving the coupled Schrodinger’s and Poisson’s equation self-consistently. Taking the piezoelectric effect into account, the two-dimensional electron gas concentration is calculated to be as high as 1019u200acm−3. To gain an understanding on how the two-dimensional electron gas distribution is influenced by dopant concentration in material, we observed the two-dimensional electron gas concentration and occupation of subbands versus doping level in GaN and in AlGaN layer. The results show that the two-dimensional electron gas concentration depends much more strongly on the doping level in AlGaN than in GaN. And besides, the heavier doping in GaN should weaken the quantum confinement in the AlGaN/GaN heterointerface.