Built-in Electrostatic Field Research Articles

Molecular-beam epitaxy of GaN/AlxGa1−xN multiple quantum wells on R-plane (101̄2) sapphire substrates

GaN / Al 0.15 Ga 0.85 N multiple quantum wells (MQWs) have been grown by plasma-assisted molecular-beam epitaxy on R-plane (101̄2) sapphire substrates. The orientation relationship was found to be (112̄0) (Al)GaN∥(10 1̄2) Al2O3, resulting in nonpolar GaN/AlGaN heterostructures. Room-temperature photoluminescence studies were performed to compare the optical properties of the MQWs grown on (0001) and (101̄2) Al2O3 substrates. The peak transition energy, as a function of well width for the (112̄0) MQWs, followed the trend for rectangular potential profiles indicating the absence of built-in electrostatic fields. In comparison, the peak transition energies for the (0001) MQWs showed a significant redshift due to the quantum-confined Stark effect, consistent with a built-in field value of 750 kV/cm. In addition, the photoluminescence intensity was 20 to 30 times higher for the (112̄0) MQWs compared to the (0001) MQWs.

Pyroelectric properties ofAl(In)GaN/GaN hetero- and quantum well structures

The macroscopic nonlinear pyroelectric polarization of wurtziteAlxGa1-xN, InxGa1-xN and AlxIn1-xN ternary compounds (large spontaneous polarization andpiezoelectric coupling) dramatically affects the optical andelectrical properties of multilayered Al(In)GaN/GaN hetero-,nanostructures and devices, due to the huge built-inelectrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces.Models of polarization-induced effects in GaN-based devices sofar have assumed that polarization in ternary nitride alloyscan be calculated by a linear interpolation between thelimiting values of the binary compounds. We present theoreticaland experimental evidence that the macroscopic polarization innitride alloys is a nonlinear function of strain and composition. We have applied these results to interpretexperimental data obtained in a number of InGaN/GaNquantum wells (QWs) as well as AlInN/GaN and AlGaN/GaNtransistor structures. We find that the discrepancies between experiment and ab initio theory present so far arealmost completely eliminated for the AlGaN/GaN-basedheterostructures when the nonlinearity of polarization isaccounted for. The realization of undoped lattice-matched AlInN/GaN heterostructures further allows us to prove the existenceof a gradient in spontaneous polarization by the experimentalobservation of two-dimensional electron gases (2DEGs). Theconfinement of 2DEGs in InGaN/GaN QWs in combination with themeasured Stark shift of excitonic recombination is used todetermine the polarization-induced electric fields in nanostructures. To facilitate inclusion of the predictednonlinear polarization in future simulations, we give anexplicit prescription to calculate polarization-inducedelectric fields and bound interface charges for arbitrarycomposition in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow adetailed comparison of the predicted electric fields and boundinterface charges with the measured Stark shift and the sheetcarrier concentration of polarization-induced 2DEGs. Thiscomparison provides an insight into the reliability of thecalculated nonlinear piezoelectric and spontaneous polarizationof group III nitride ternary alloys.

Role of polarization in the photoluminescence of C- and M-plane oriented GaN/AlGaN multiple quantum wells

ABSTRACTThe comparative study of photoluminescence (PL) dynamics of wurtzite-type GaN/AlGaN multiple quantum wells (MQWs) fabricated using low-pressure metalorganic chemical vapor deposition technique over GaN coated [0001]-sapphire (C-plane) and single crystalline [1100]-oriented freestanding GaN (M-plane) substrates is presented. The MQWs on C-plane sapphire at low excitation exhibited much lower (∼30 times) PL intensity in comparison with M-plane samples. The C-plane MQWs showed a strong excitation intensity-induced PL spectrum line blueshift (up to 140 meV). Meanwhile identical MQW structures on M-plane substrates demonstrated no PL peak shifts indicating an absence of polarization fields. At higher excitation (>50 kW/cm2) the PL intensity and spectra peak positions for both the C- and the M-plane MQWs become nearly the same and do not differ with subsequent increase of pumping. Theoretical analysis and comparison with PL experimental data revealed strong (up to ∼1.2–1.3 MV/cm) built-in electrostatic fields in the C-plane structures whereas M-plane structures are almost non-polar.

Spontaneous versus Piezoelectric Polarization in III-V Nitrides: Conceptual Aspects and Practical Consequences

Macroscopic polarization plays a major role in determining the optical and electrical properties of nitride nanostructures via polarization-induced built-in electrostatic fields. While currently fashionable, this field of endeavour is still by far in its early infancy. Here we contribute some clarifications on the conceptual issues involved in determining built-in fields in III-V nitride nanostructures, sorting out in particular the roles of spontaneous and piezoelectric polarization.

Effects of Impurity Deionization and Temperature on Carrier Equivalent Lifetime in Diffused Semiconductor Structures

In an inhomogeneously doped semiconductor built-in electrostatic field and mobility gradients result in a minority-carrier equivalent lifetime which is of a few orders lower in magnitude compared to normal lifetime in a homogeneous semiconductor. To find the built-in field it is commonly assumed that all the impurity atoms are fully ionized at room temperature. Since this assumption is not valid in some cases, expressions have been derived for the built-in field and the equivalent lifetime taking deionization into account. It is shown that the consideration of deionization for an exponential distribution results in a nonconstant built-in field. Second, the temperature variation of minority-carrier equivalent lifetime has been studied. To make an analytical study possible, an empirical relation for the minority-carrier mobility in silicon as a function of both temperature and impurity concentration has been proposed. Some typical inhomogeneous silicon layers having exponential, Gaussian, and complementary error-function impurity distributions have been considered. It is shown that the equivalent lifetime may decrease with temperature in some regions of an inhomogeneous layer, contrary to the deductions obtained from the Shockley-Read theory for normal lifetime.

Carrier profiles and collection efficiency in Gaussian p-n junctions under electron beam bombardment

Gaussian-doped semiconductor junctions have a built-in electrostatic field E which usually varies linearly with distance from the surface. A series of solutions of the one-dimensional continuity equation for linear E field are given in terms of the confluent hypergeometric function and its limiting forms. These solutions are directly applicable to the collection of hole-electron pairs generated between the surface and the junction, and can be extended readily to cover generation at greater depths. The effects of bulk recombination and spatial variation of diffusion constant on the theoretical collection efficiency are also calculated. Using a generation function appropriate to a kilovolt electron beam, the minority carrier density versus distance and the collection efficiency of the junction are evaluated for zero and infinite surface recombination velocity.

The Built-In Electrostatic Potential, Field, and Field Gradient in Diffused p-n Junctions

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Effect of Dimensions on the Efficiency of Radiant Energy Cells

This paper deals with the enhancement of the quantum efficiency and photovoltaic energy conversion efficiency of a p-n semiconducting cell by optimizing the dimensions of the cell. Based on the Shockley-Read statistics a general expression for the quantum efficiency of monochromatic incident radiant energy photons has been derived in terms of the absorption coefficient of the incident photons, the minority carrier diffusion length, the built-in electrostatic field appearing in diffused cells, and the surface recombination velocity in the exposed layer of the cell. Although the expressions derived may be used for all semiconducting p-n cells, special efforts have been made in the analysis and computations of the germanium p-n cell. The germanium cells show a great potential for photovoltaic energy conversion from radiant sources other than the sun. The results for germanium indicate that the quantum efficiency strongly depends upon the thicknesses of the exposed and base layers. The built-in electrostatic field and the surface recombination velocity in the exposed layer influence the quantum efficiency greatly. Optimization studies for the thicknesses of the exposed and base layers of an n-p type germanium for different values of minority carrier diffusion length, built-in electrostatic field, and surface recombination velocity have been worked out.