III-nitride structures and devices with quantum wells and superlattices

Current research focus
Photoluminescence of extended structural defects in semiconductors
Misfit dislocations and radiative efficiency of In_xGa_1-xN/GaN quantum wells
Charge Asymmetric Resonance Tunneling (CART) as a basic element of GaN based light-emitting diode
III-nitride Unipolar Light Emitting Devices
Physics of surface of elastically stressed solids
Temperature behavior of surface conductivity in Si with structural defects
Contact information

Current research focus

The primary focusof our Research group work is on physics of extended defects in III-N nanostructures. Our current research interest in study of extended defects in GaN is greatly stimulated by rapid progress in the development of III-nitride optoelectronics.
These compound semiconductor materials have the huge density of structural defects, but highly efficient blue/green GaN-based light emitting diodes (LEDs) are successfully commercialized in spite of high density of dislocation density (10¹⁰ - 10¹¹ cm^-2).
However, dislocations degrade the performance and reliability of LEDs, form the leakage current pathway in quantum wells, led to increasing of threshold and operating currents in GaN-based violet and blue laser diodes, limit device lifetime. It was found recently that threading dislocations with a screw component act as a strong of non-radiative centers in GaN. Mixed dislocations are found to led to phase separation in InGaN quantum well. The lack of understanding of the defect nature in III-nitride-based devices and their role in light emission mechanism is still exist.
Thus, an understanding of the structure and electronic properties of the defects in these materials is essential not only from fundamental point of view, but also for improving material quality, device design and performance.

The research activity of the Group includes:

• Study of recombination, transport, defects in III-N materials and devices
• Design of novel light-emitting devices based on quantum wells and superlattices
• Theoretical modeling of device structures

Another direction of our group efforts is structural defects formed on surface of solids under mechanical stress. The fundamental mechanisms of destruction of elastically stressed materials at thermal and chemical activation of their surface are investigated. The kinetic mechanism of surface instability evolution during etching, corrosion, and growth is investigated. The role of stacking faults, dislocations and artificially created of surface steps in corrosion development of stressed Si crystals is studied.

Photoluminescence of extended structural defects in semiconductors

One of the semiconductor material science problems is the nature of electronic states, which are introduced by dislocations, or stacking faults. Recently we have suggested a deformation potential model to analyze electronic and optical properties of the extended defects. According to this model, the one-dimensional energy bands split off from the edges of the bulk bands by the strain field of the defects. Accordingly, the energy position of the dislocation-related photoluminescence is determined by the edge component of the dislocation Burgers vector.

The principal results which have been obtained using this approach are:

• the spectral lines of the main extended defects in Si were classified according to the Burgers vector
• dislocation excitons were found and studied in Ge, Si, ZnSe and GaN crystals
• electrically-detected spin resonance of the electrons on the 60 degree -dislocations was observed in Ge⁷⁴ - enriched n-Ge crystals
• it was shown that stacking faults in GaN are natural quantum wells for electrons. Stacking fault-bound excitons were found in GaN/SiC films

Misfit dislocations and radiative efficiency of In_xGa_1-xN/GaN quantum wells

The most common defects in wurtzite GaN epilayers grown on sapphire are threading dislocations originated from high-lattice-mismatch between the epilayer and substrate. These defects can significantly affect the nonequilibrium carrier lifetime at low injection levels and prevent low laser thresholds. Recently, dislocations of another type, localized in the In_xGa_1-xN quantum wells, have been observed, which most probably are the misfit dislocations at GaN/In_xGa_1-xN interface. We investigate the influence of the misfit dislocations at the interface of the In_xGa_1-xN quantum well on the radiative efficiency of the carriers in quantum well.

It was found that misfit dislocations with density up to ~ 105 - 106 cm-1 could improve the quantum efficiency of the InxGa1-xN quantum wells by more than 10 times because they reduce the quantum well built-in electric field. At higher densities, the misfit dislocations suppress the quantum efficiency of the wells since they produce an additional channel of nonradiative recombination.

1. Stepanov S.I.; Wang W.N, B.S.Yavich, V.E.Bougrov, Y.T.Rebane,Y.G.Shreter, Influence of Poisson's ratio uncertainty on calculations of the bowing parameter for strained InGaN layers. The Materials Research Society Internet Journal of Nitride Semicondors. 2001, 6, p.1-6
2. Y.T.Rebane, Y.G.Shreter, W.N.Wang, Misfit dislocations and radiative efficiency of InxGa1-xN/GaN quantum wells. Applied Surface Science 166, 300 (2000)

Charge Asymmetric Resonance Tunneling (CART) as a basic element of GaN based light-emitting diode

We suggest a system of two wells with Charge Asymmetric Resonance Tunnelling (CART) as a basic element of light-emitting diode (LED) structure for semiconductors with different masses of electrons and holes.

The system consists of an emitter of the electrons, an emitter of holes and an active region. The hole emitter is coupled with the active region in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is designed as a wide well and is coupled to the active layer via a barrier.

The barrier design uses the charge asymmetric resonance-tunnelling phenomenon, which allows to make the barrier transparent for electrons and blocking for heavy holes. The phenomenon of the charge asymmetric resonance tunnelling uses the quantum mechanical effect of strong exponential dependence of the potential barrier tunnel penetrability on the mass of the tunnelling particle. First results of experimental investigation and theoretical modeling of the CART LED are obtained.

EL spectra at 20 mA from non-optimised (left) and optimised (right) CART structures. In the non-optimised structure, wide-well recombination occurs. In the optimised structure, only QW-related emission is observed.

III-nitride Unipolar Light Emitting Devices

Unipolar light emitting device based on III-nitride superlattices for the generation of visible light is suggested. The main idea of the unipolar light emitting device (U-LED) is to create the analogue of an n-p junction between two n-type superlattices with a shallow and a deep subband.

The superlattice with the shallow subband acts as an effective n-type semiconductor, whereas the superlattice with deep subband plays the role of an effective p-type semiconductor. The radiation arises due to the electron transitions from the shallow subband superlattice into the deep subband superlattice. The conduction band off-set between AlN and InN is about 3 eV and this value can be reduced using the alloys of AlGaN.

This allows one to get electron transitions between two superlattices based on these alloys with energies in the range of 0 to 3 eV, which covers the visible and the infrared range of the spectra.
The quantum efficiency of these transitions could be enhanced by inserting between two superlattices some optically active layer with two quantum states , which can be a specially designed quantum well, impurity layer or quantum dot layer. A significant increase in the efficiency can be achieved with the use of the active layer doped with deep acceptors. In this case the optical transitions take place from the quantum well subband of the active layer into the deep acceptor impurity band. The transition metals Fe and Ni are considered as possible deep acceptors for the active layer in GaInN/AlN superlattices.

1. Y.G.Shreter, Y.T.Rebane and W.N.Wang. III-Nitride Unipolar Light Emitting Devices, Phys.stat.sol.(a) 180, 307 (2000)

Physics of surface of elastically stressed solids

Corrosion precursors in the form of microgrooves appearing on the elastically compressed surface of a silicon plate under etching are observed and investigated. No corrosion precursors are observed on the elastically stretched surface. This distinguishes the observed effect from corrosion cracking of metals, during which corrosion usually takes place on stretched surfaces.

The general dynamic model proposed for the evolution of surface microgrooves during etching, corrosion, and growth of elastically stressed solids is based on the concept of two local etching (growth) rates which are linear functions of the local stress tensor. The model describes the kinetics of the process, and the asymmetry of corrosion evolution to the deformation sign. The role of stacking faults, dislocations, and artificially created surface steps in the evolution of corrosion in stressed silicon crystals is studied.

For the first time the influence of magnitude and sign of elastic strain in a structures of crystal evaporated metal film on form and sizes of forming metal droplets and distances between droplets is found. The dependence of droplet sizes and distances between droplets on degree of deformation is observed at the stretched surface in the structure Si - In.

Temperature behavior of surface conductivity in Si with structural defects

Despite many years of research, the nature of Si/SiO₂ interface states remains controversial. It is known that oxygen- and hydrogen-induced defects have dramatic and complex effects in the Si-SiO₂ system. It is clear that they also can play important role in nanostructures due to gettering at interfaces and dislocations. We studied surface conductivity in crystals of Ge and Si with structural defects using Schottky diodes with surface leakage channels. A set of sharp peaks and steps in the temperature curves of HF conductance and capacitance was observed. It was shown that all of the features are due to low-temperature changes of surface conductivity and reflect a set of interface states. These features were observed in Ge and Si at similar temperatures. The results compared with known data on behavior of hydrogen and oxygen in nanopores and at the surface of solids. It is concluded that intrinsic properties of oxygen- and hydrogen-containing complexes to rearrange the structure at low temperatures with change of the charge are responsible for observed features. We believe that the tendency to donor action of oxygen complexes due to increase of molecular bonds and hole self-trapping at low temperatures is the cause of low-temperature doping of near-surface layer of crystals.

Schottky diodes on Si with oxidation-induced stacking faults are studied. The surface channels on their periphery are formed due to native or thermal SiO2. The temperature dependences of the conductivity (G), capacitance (C) and transient capacitance spectra (Delta C) are measured. It is concluded that the surface density of free electrons gradually increases upon cooling from 200 to 80 K at "intrinsic" temperatures of oxygen subsystem of SiO2. The dynamics of the channel-current response to the voltage change is associated with dispersive hopping transport of holes in SiO2.

References

1. Y.G.Shreter, Rebane Y.T., Klyavin O.V., Aplin P.S., Axon C.J., Young W.T. and Steeds J.W. Dislocation-related absorption and photoluminescence in deformed n-ZnSe crystals. J.Crystal Growth, 159, (1996) 883.
2. Y.T.Rebane and Y.G.Shreter Nature of dislocation-related luminescence in diamond, zinc-blend and wurtzite-type semiconductors . Inst.Conf.Ser. No 155 pp 703-706 (1997)
3. P.Fernandez, J.Piqueras, A.Urbieta, Y.T.Rebane and Y.G.Shreter Deformation induced defect levels in ZnSe crystals. Semicond.Sci.Technol. vol 14, pp 430-434 (1999).
4. Stacking fault as quantum wells for excitons in wurtzite GaN. Y.G.Shreter, Y.Rebane and M.Albrecht. Phys.Stat.Sol., (a) 164, 141 - 144(1997)
5. Kinetics of electric field screening in a space-charge region with a leakage channel and low-temperature conductance of surface channels in high-resistivity n-Si. N. I. Bochkareva and A. V. Klochkov . Semiconductors, 33, pp. 1212-1215, 1998

Phone: +7 (812) 247 9152, +7 (812) 247 3256
Fax: +7 (812) 247 1017, +7 (812) 247 3256
E-mail: Shreter@peterlink.ru

Nanoscale Structures and Devices Group
Laboratory of Nonequilibrium Processes in Semiconductors
Ioffe Physico-Technical Institute, Russian Academy of Sciences
Politekhnicheskaya 26,
St.Petersburg, 194021 Russia