Strong Built-in Electric Field Research Articles

The donor bound exciton states in wurtzite GaN quantum dot

Based on the effective mass approximation, the donor bound exciton states in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) are investigated by means of a variational method, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the donor bound exciton binding energy is highly dependent on the impurity position and QD size. In particular, we find that the donor bound exciton binding energy is insensitive to dot height when the impurity is located at the right boundary of the WZ GaN/AlGaN QD with large dot height.

Exciton states in wurtzite InGaN/GaN quantum wells: Strong built-in electric field and interface optical-phonon effects

Considering the strong built-in electric field (BEF) effects and large exciton–phonon interactions, we investigate the exciton states confined in an InGaN/GaN single quantum well (QW) by using the Lee–Low–Pines variational method. We find that the exciton state modification caused by the exciton–phonon interactions is remarkable. The exciton energy shift due to exciton–phonon interactions increases monotonically if the well width increases. With increasing the In fraction, the exciton energy shift firstly increases to a maximum, then decreases. The BEF has a significant influence on the exciton states in a QW with large well width. The physical reasons have been analyzed in detail. Good agreement for the zero-phonon peak energies and the Huang–Rhys factor has been obtained between our numerical results and the corresponding experimental measurements.

THIRD-HARMONIC GENERATION OF WURTZITEGaN-BASED COUPLING QUANTUM WELLS WITH STRONG BUILT-IN ELECTRIC FIELDS

Based on the density matrix approach and iterative treatment, the third-harmonic generation (THG) susceptibility of a wurtzite nitride coupling quantum well (CQW) with strong built-in electric fields have been theoretically investigated. The band non-parabolicity effect of nitride heterostructures has been taken into account. A typical wurtzite AlGaN/GaN CQW are chosen to perform numerical calculations. The result shows that, the THG coefficients sensitively depend on the structural parameters of the CQW system. Moreover, a strong THG process can be realized in the nitride CQW by choosing a group of appropriate structural parameters and doped fraction.

Linear and Nonlinear Intersubband Optical Absorptions andRefractive Index Changes in InGaN Strained Single Quantum Wells: StrongBuilt-inElectric Field Effects

Considering the strong built-in electric field (BEF) effects due tothe spontaneous and piezoelectric polarizations, the intersubbandoptical absorptions and refractive index changes for anInxGa1−xN/AlyGa1−yN strained single quantum well arestudied theoretically within the framework of the density matrixmethod and effective-mass approximation. The linear, third-ordernonlinear and total absorption coefficients and refractive indexchanges are calculated as a function of the incident opticalintensity and photon energy. Our results show that both the incidentoptical intensity and the strong BEF have great influence on thetotal absorptions and refractive index changes. The results aresignificant for designing some important photodetectors and thephotonic crystal devices with adjustable photonic band structures.

Hydrogenic donor states in wurtzite InGaN/GaN coupled quantum dots

The binding energies of the hydrogenic impurity in wurtzite InGaN coupled quantum dots (QDs) are calculated by means of a variational method, considering the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetrical distribution of the donor binding energy with respect to the center of the coupled QDs. When the impurity is located in the center of the left dot, the donor binding energy is largest and insensitive to the barrier height of the wurtzite InGaN coupled QDs.

Influence of the built-in electric field on luminescent properties in self-formed single-GaN/Al xGa 1−xN quantum dots

Based on the framework of effective-mass approximation and variational approach, the luminescent properties are investigated theoretically in self-formed wurtzite GaN/Al x Ga 1− x N single-quantum dots (QDs). Considering the three-dimensional (3D) confinement of electron and hole pair and the strong built-in electric field effects, the exciton binding energy, the emission wavelength and the oscillator strength are calculated with and without the built-in electric field in detail. The results elucidate that the strong built-in electric field has a significant influence on luminescent properties of GaN/Al x Ga 1− x N QDs.

Parameter-dependent third-order susceptibility of InxGa1−x N/GaN parabolic quantum dots

The electron states confined in wurtzite InxGa1−xN/GaN strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrodinger equation, in which parabolic confined potential and strong built-in electric field effect (due to the piezoelectricity and spontaneous polarization) have been taken into account. The real part Reχ(3) (0,0,ω) and the imaginary part Im χ(3) (0,0,ω) of the third-order susceptibility deseribe quadratic electro-optic effects and electro-absorption process of the QDs respectively. And both of them have been calculated in directions parallel and vertical to z axis. Furthermore, the study shows Re χ(3) (0,0,ω) and Im χ(3) (0,0,ω) increase under resonant conditions with the QDs’ radius and height increase, and the same results occur when the content increase. In addition, the resonant position shift to the lower energy region when the parabolic frequencies increase.

Second-order nonlinear optical susceptibilities induced by built-in electric field in wurtzite nitride double quantum wells

Based on the density matrix method and the iterative treatment, the second-harmonic generation (SHG) susceptibility of a wurtzite nitride coupling quantum well (CQW) with strong built-in electric fields have been theoretically investigated. The effect of the band non-parabolicity effect has been taken into account. A typical wurtzite GaN/InxGa1–xN CQW are chosen to perform numerical calculations. The localized properties of the electronic ground state and the low-excited states in the system are analyzed in detail. The calculated SHG coefficients reach the order of magnitude of 10−7m/V, which is two-order larger than the corresponding values in wurtzite single quantum wells. Moreover, it is confirmed that the SHG coefficients are not monotonic functions of the well width, barrier width and the doped concentration of the CQW systems, but have complicated dependent relations on them. The reasons resulting in these characteristics can be attributed to the intense competition between the strong built-in electric field effect and the quantum size effect for the electronic confined situation in the wurtzite CQWs. The calculated results also show that a strong SHG effect can be realized in the nitride CQW by choosing a group of optimized structural parameters and doped fraction.

Built-in electric field effect on hydrogenic impurity in wurtzite GaN/AlGaN quantum dot

The binding energy of a hydrogenic donor impurity in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) is investigated, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetrical distribution of the donor binding energy with respect to the center of the QD. The donor binding energy is insensitive to dot height when the impurity is located at the right boundary of the QD with large dot height.

NONLINEAR OPTICAL RECTIFICATION OF A COUPLING WURTZITEGaN-BASED QUANTUM WELL WITH BUILT-IN ELECTRIC FIELDS

Taking the strong built-in electric field into consideration, the optical-rectification (OR) coefficient in a nitride semiconductor coupling quantum well (CQW) has been theoretically investigated by using the compact density matrix approach. The electronic eigenstates in a nitride CQW are exactly solved based on the built-in electric field model already constituted in the recent reference. Numerical calculations on the typical GaN / InxGa1-xN CQW are performed. The calculated results reveal that the OR coefficients of the CQW are not monotonic functions of the well width, barrier width, and the doped concentration of the CQW systems but have complicated dependent relations on them. Our calculation shows that a strong OR effect can be obtained in the nitride CQW by choosing optimized structural parameters and a relatively low doped fraction.

Parameter-dependent third-order nonlinear susceptibility of parabolic InGaN/GaN quantum dots

The electron states confined in wurtzite InxGa1−xN/GaN-strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrödinger equation, in which parabolic confined potential and strong built-in electric field effect due to the piezoelectricity and spontaneous polarization have been taken into account. The third-order nonlinear susceptibility of the QDs in various directions (both parallel to z direction and vertical to z direction) have been calculated, and the magnitude reaches 10−14m2/V2. It has been shown from the results that the order of the built-in electric field in the strained QD is of MV/cm. Furthermore, the results of how the third-order nonlinear susceptibility depend on the radius R of QDs, the height L of QDs, the In content x of QDs and the relaxation rate Γ10 have been given.

Radiative lifetime in wurtzite GaN/AlN quantum dots

We study wurtzite GaN/AIN quantum dots (QDs) by time-resolved photoluminescence. The properties of nitride based nano-objects are significantly affected by strong built-in electric fields existing in this crystalline phase. These fields induce a spatial separation of electrons and holes in the quantum dot, a Stark shift of the transition energy, and they slow their radiative recombination down to tens of microseconds of decay time for the largest quantum dots. Such long decay times are responsible for the screening of internal fields by using moderate excitation intensities, thus modifying drastically both the energy and the rate of photon emission. Consequently it is difficult to perform spectroscopic studies of these QDs in the regime where a single electron-hole (e-h) pair is present at the most in each dot. Using a pulsed laser excitation, we study the time-resolved dynamics of carrier recombinations, waiting for complete recombination of all electron-hole pairs in the dots. We clearly exhibit the conditions where no screening of the internal electric field occurs. We especially obtain the transition energy and the mono-exponential decay, i.e the radiative recombination time, of the last e-h pair present in the dot, for a wide range of QD sizes. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0 +/- 0.5 MV/cm.

Energy difference between electron subbands in AlInN∕GaInN quantum wells studied by contactless electroreflectance spectroscopy

Contactless electroreflectance (CER) spectroscopy has been applied to study the energy difference between electron subbands in AlInN∕GaInN multiquantum wells (MQWs). Due to the strong built-in electric field, which is typical for III-nitrides, interband transitions between all QW subbands have been clearly observed. The energy difference between electron subbands has been found analyzing the interband transitions between the first hole subband and electron subbands. It has been shown that this difference is consistent with the results obtained by using photoinduced absorption of intersubband transitions. It shows that CER is an excellent method to study the energy difference between electron subbands in Al(In)N∕Ga(In)N QWs.

Advances in growth and optical properties of GaN‐based quantum dots

AbstractGaN‐based quantum‐dot systems have received great attention and revealed unique zero‐dimensional features of localized excitons in those quantum dots. In this article, we review recent advances in growth and optical properties of the GaN‐based quantum dots focusing on hexagonal GaN quantum dots, which are unique in terms of the existence of a strong built‐in electric field due to a spontaneous polarization and a piezoelectric field. In addition, we also discuss their potential applications to advanced light‐emitting devices, such as lasers and single‐photon sources. Especially, the application to quantum information devices including single‐photon sources with a GaN quantum dot is a promising direction because the GaN quantum dots have the potential for operation at higher temperatures. For such applications to quantum information technology, fine tuning and controlling of the electronic states are indispensable, which are generally performed via the quantum‐confined Stark effect. We have demonstrated thelarge tunability of the energy level in a GaN quantum dot by externally applying an electric field. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Built-in electric field effects on donor bound excitons in wurtzite InGaN strained coupled quantum dots

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations of the wurtzite In x Ga 1− x N/GaN strained coupled quantum dots (QDs), the positively charged donor bound exciton binding energy is calculated within the framework of the effective-mass approximation and variational method. Our results clearly indicate that the donor bound exciton binding energy sensitively depends on the donor position in the system, the structural parameters of the coupled QDs and the strong built-in electric field. The variation of this energy versus the donor position is in several tens of meV.

Impurity bound polaron in wurtzite GaN/AlN quantum wells: The interface optical-phonon and the built-in electric field effects

Considering the interaction of electrons and impurities with interface optical (IO) phonons , the ionization energy of a bound polaron in wurtzite GaN/AlN strained quantum wells (QW’s) is investigated theoretically by means of a modified Lee–Low–Pines variational approach, in which the strong built-in electric field (BEF) due to the spontaneous and piezoelectric polarizations of GaN/AlN QW’s is included. Our numerical calculations show that the interaction between the impurity and the IO-phonon field plays an important role in screening the Coulomb interaction. Moreover, we find that the BEF has a considerable influence on the bound polaron effect in GaN/AlN QW’s with a large well width. The ionization energy of a bound polaron sensitively depends on the position of the impurity center in GaN/AlN QW’s. With the increasing of the well width d ( d > 2.5 nm ), the donor ionization energy increases monotonically and approaches a constant for the case of the donor fixed on one interface of the QW due to the BEF. On the contrary, the ionization energy decreases slowly with the increasing of the well width ( d > 0.25 nm ) if the BEF is ignored.

Interband Optical Transitions due to Donor Bound Excitons inWurtzite InGaN Strained Coupled Quantum Dots: Strong Built-in ElectricField Effects

Considering the three-dimensional confinement of the electrons and holesand the strong built-in electric field (BEF) in the wurtziteInGaN strained coupled quantum dots (QDs), the positively chargeddonor bound exciton states and interband optical transitions areinvestigated theoretically by means of a variational method. Ourcalculations indicate that the emission wavelengths sensitivelydepend on the donor position, the strong BEF, and the structureparameters of the QD system.

Hydrogenic impurity states in a wurtzite InGaN quantum dot

Within the framework of effective-mass approximation, we calculated variationally the binding energy of a hydrogenic donor impurity in a cylindrical wurtzite (WZ) InGaN/GaN strained quantum dot (QD), including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. It is found that the donor binding energy is highly dependent on the impurity position and QD size. In particular, we found that the donor binding energy is independent of dot height when the impurity is located at the right boundary of the WZ InGaN strained QD with large dot height ( ⩾ 2.5 nm ).

Influence of the built-in electric field on luminescent properties in self-formed single In xGa 1−xN/GaN quantum dots

Based on the framework of effective-mass approximation and variational approach, luminescent properties are investigated theoretically in self-formed wurtzite In x Ga 1− x N/GaN quantum dots (QDs), considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the height L and the radius R) are calculated with and without the built-in electric field in detail. The results elucidate that the strong built-in electric field has a significant influence on luminescent properties of In x Ga 1− x N/GaN QDs.

Donor bound excitons in wurtzite InGaN quantum dots: Effects of built-in electric fields

Within the framework of the effective-mass approximation and variational approach, we present calculations of the bound exciton binding energy, due to an ionized donor, in wurtzite In x Ga 1− x N/GaN strained quantum dots (QDs), considering three-dimensional confinement of the electron and hole in the QDs and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Our results show that the position of the ionized donor, the strong built-in electric field, and the structural parameters of the QDs have a strong influence on the donor binding energy. The variation of this energy versus position of the donor ion is in double figures of milli-electron volt. Realistic cases, including the donor in the QD and in the surrounding barriers, are considered.